Question
Why Jesus Isu Christ, Guru Nanak, Buddha and Muhammad should not be considered as Lord Vishnu’s avatars?
Answer
You have realised the correct thing. But the reason they should not be considered as avatars of Vishnu is because it has not been explicitly mentioned in the scriptures.
It is the scriptures which tell us in the first place who Vishnu is and what are His incarnations. So without the scriptures explicitly mentioning any of the prophets it will always remain a subjective thing to decide whether someone is an avatara of Vishnu or not. It's because the scriptures, even after mentioning the major incarnations of Vishnu, don't forget to mention that His incarnations are nothing but infinite (avatārā hyasaṃkhyeyā hareḥ [SB - 1.3.26])
People are driven by ego
So your assertion that the founder or prophets of other major religions must be avatars of Vishnu is correct, but it's subjective. Undoubtedly God is one whether we call Him Vishnu or something else, but people's ego is attached to whatever things they believe that they cannot think out of the box and feel everyone else is wrong. But who realise and understand the message of the scriptures know that God is equally in all and impartial to everyone (samoham sarva bhutesu) irrespective to their cast, creed and religion. But because not all people realise this, we see much hatred and negative feelings between strong adherents of different religions. What to say about religions, we even see hatred between followers of different sects in the same religion. All because of ego and lack of correct knowledge. With correct knowledge one realises everyone and everything as Vishnu Himself, not only just the prophets. For people with the correct knowledge everything is Brahman (sarvamkhalu idam brahman):
bahunam janmanam ante jnanavan mam prapadyate
vasudevah sarvam iti sa mahatma su-durlabhah [BG - 7.19]Meaning
After many births and deaths, he who is actually in knowledge surrenders unto Me, knowing Me to be the cause of all causes and all that is. Such a great soul is very rare.
God descends to establish religion
Like the verse from Gita (yada yadahi..) you have mentioned says, God descends to establish religion. Also, He makes people nimmita (medium) for establishing His desire. For example, as Krishna He made Arjuna a nimmita to kill the unrighteous Kauravas.
So certainly, the task of establishing the other religions were done by His people whom we call as prophets. They can be considered as ansha avataras (partial incarnations). But again considering this is only subjective. There is Bhavisya Purana which mentiions this, but it is not considered as completely authentic due to the fault of possible later interpolations. I have some personal findings from which I know Jesus is a part incarnation of God, but I wouldn't share them because they can be subjective and confusing.
Without God's will not even a leaf moves
Exactly. So it is only under His will that religions of different kinds which are even contradictory to each other has come into existence. Hence, whether someone is a follower of Sikhism, Buddhisim, Jainsim or any other Abrahamic religion, in everyone's heart the same God resides equally and moves them through His maya (illusory potency):
isvarah sarva-bhutanam hrd-dese 'rjuna tisthati
bhramayan sarva-bhutani yantrarudhani mayaya [ BG - 18.61]Meaning
The Supreme Lord is situated in everyone’s heart, O Arjuna, and is directing the wanderings of all living entities, who are seated as on a machine, made of the material energy.
So you can say, it is the same Lord who is pulling everyone's string. That is, by making people nimmita He is causing different kinds of things.
Bottom line
So the conclusion is, your personal realisation is correct but because there is no authentic scriptural testimonies to support them being the avatars of Vishnu, you can't just claim it out in the open. Even if you get scriptural testimony, people will ridicule, mock and say it's a later interpolation. We have plenty of scriptural testimony for Buddha being an avatar of Vishnu, but even then people have differing views. Some say those are later interpolations and accept Balarama as an avatar of Vishnu instead of Buddha. Now how can one make someone understand when he rejects the scriptures in favor of his personal views? Hence, because people will always have differing views depending upon their level of knowledge and understanding, it's nice to have realisations but not so nice to try to make others realize.
Update
So after reading your comments it seems you are trying to find reasons to not consider prophets of other religion as avatars of Vishnu instead of the fact that they did the work of establishing religions which is done only by God or His chosen people. But unfortunately considering someone as an avatara or not is only subjective unless it is mentioned in scriptures which are accepted as the standard.
Avatarati iti avatara. That is, as the lord descends down to earth as a mere mortal from His higher position, He is called as an avatara. Generally the avataras are categorised as three kinds: purna avatara, avhesha avatara and amsa avatara, but this is not the standard and various other types exist as well. So the point is, generally the major avataras are listed in the scriptures and the numerous amsa avataras are not listed in them. So recognising these amsa avataras become difficult and subjective. I am giving an example below.
Avatara Case Study
Around 515 years ago Nimai pandit (Chaitanya Mahaprabhu) appeared and spread the nama samkirtana (chanting lord's names) movement. Nama Samkirtana is the prescribed yuga dharma of the scriptures for the age of kali. He is considered as an incarnation of Radha-Krishna by the followers of Gaudiya Vaishnavism. But there is no direct mention of Him in the scriptures. So considering Him an avatara is completely subjective. Hence, followers of other sects don't consider Him as an avatara. He even showed a sadabhuja (six armed) divine form probably to Svarupa Damaodara. There were many such incidents, but who can validate those?
Miracles are not standard
Miracles like showing four armed form is not a standard because doing so will throw faith out of the window, and not doing so will also cause disbelief. So all saints at some point or other showed some form of miracle be it bringing the dead back to life or turning the water into wine. Also, in course of history many charlatans showed cheap miracles to prove their divinity.
Physical features are not standard
It is said that an avatara of Vishnu will have the marks of sankha (conch shell), Chakra, gada (mace), etc. While I believe this because I know it from a trust worthy source, there are other physical features like height, hair, teeth, etc. that are considered by some which cannot be the standard.
Six standard qualities that the Lord posses
According to the scripture the lord or Bhagavan should have the following six qualities:
aiśvaryasya samagrasya dharmasya yaśasariśrayaḥ
jñānavairāgyayoścaiva ṣaṇṇāṃ bhaga itīraṇā [VP - 6.5.74]Meaning
Complete splendor, virtue, glory, opulence, knowledge, dispassion - these six are known as bhaga.
So by this standard, if someone doesn't have these six qualities then he is not an avatara. But if someone has all these then you have to consider him as an avatara. Shri Krishna qualifies this standard and hence in Gita it is written Bhagavan Uvacha and so also in many other scriptures. However, the problem with this standard is that it works only with purna avataras, i.e. the incarnation in which all the opulences of the Lord manifests. But in amsa avataras (partial incarnations) all the qualities may not be there.
So without some standards it is always subjective to recognise an amsa avatara unless one takes the words of a saint or scripture for granted. You want reasons for not considering them as avataras, but unfortunately my understanding is different. They are partial incarnations of the lord. How do I know? I can only say that I take the words of a saint for granted.
But unfortunately in course of time there also have been many false saints who are considered as an avatara by their followers. So it is a tricky task. But from my personal experience, all that I can say is that if one has genuine faith and devotion in Lord then he will certainly find out what is right even if he has to undergo and believe what is wrong.
People have to wait until Kalki's appearance?
Kaliki is a major avatara who is said to appear towards the end of the age of kali. You feel that people have to wait until He appears and they will not try to be righteous. But to uphold dharma saints and sages will take birth time to time and encourage people. They will do the work as nimiita (medium) just like you had said. So there is nothing to be worried about. Nevertheless, towards the end evil and adharama will certainly rise to a level that the Lord will have to descend in the house of Vishnujasa in Sambala village.
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