Why is Muhammad (SAW) the final prophet unlike what is believed by people who are still following other Abrahamic religions?
Actually, it is obvious to every human being who thinks well.
Entering Christianity now denies Quran and more than thousands of hadiths (so much text).
Entering Judaism now denies even much more text.
Entering Islam now doesn't deny any of the previous religions even if differences exist; because the original books are no longer there, which means there is a chance that they have been altered, and trinity of Christianity is mentioned in Quran as a false belief:
O People of the Scripture, do not commit excess in your religion1 or say about Allāh except the truth. The Messiah, Jesus the son of Mary, was but a messenger of Allāh and His word which He directed to Mary and a soul [created at a command] from Him. So believe in Allāh and His messengers. And do not say, "Three"; desist - it is better for you. Indeed, Allāh is but one God. Exalted is He above having a son. To Him belongs whatever is in the heavens and whatever is on the earth. And sufficient is Allāh as Disposer of affairs.
If you somehow still think that this huge amount of text can be denied, the challenge of Quran can't be, it will forever remain a challenge:
Say, "If mankind and the jinn gathered in order to produce the like of this Qur'an, they could not produce the like of it, even if they were to each other assistants."
Or do they say, "He invented it"? Say, "Then bring ten surahs like it that have been invented and call upon [for assistance] whomever you can besides Allah , if you should be truthful."
And if you are in doubt about what We have sent down upon Our Servant [Muhammad], then produce a surah the like thereof and call upon your witnesses other than Allah , if you should be truthful.
Or do they say [about the Prophet], "He invented it?" Say, "Then bring forth a surah like it and call upon [for assistance] whomever you can besides Allah , if you should be truthful."
And there are many scientific facts including about quantum mechanics. Learn before you judge the Quran.
The following ayah shows that Islam is the only religion to be accepted by Allah:
And whoever desires other than Islām as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers.
The following hadith shows that Islam is the final religion:
Sahih al-Bukhari 3535
Allah's Messenger (ﷺ) said, "My similitude in comparison with the other prophets before me, is that of a man who has built a house nicely and beautifully, except for a place of one brick in a corner. The people go about it and wonder at its beauty, but say: 'Would that this brick be put in its place!' So I am that brick, and I am the last of the Prophets."