You have got it bass-awkward. During the 18th century, many figures of the Enlightenment began cherry-picking among the figures they knew from classical writings who were famous for resembling Jesus in one way or another in order to make an assertion like yours. Problem is the surviving writings we have from the time of Jesus are maybe 5% of the total. Many, many figures we know nothing about except what appears as an aside in a surviving work. Ancient history is really just a patchwork of conjectures based on what does survive. But apart from this, there is no reason why a lot of people outside the Jesus movement would know or write anything about him. What remains, however, is more than what we have about Mohammed from non-Muslim sources, which is nada.
I doubt it is anywhere near 5% to tell the truth. Back in those days paper was expensive and they would be erased and reused quite often. What is found are usually scraps too.