Many Christians accept that a historical Jesus existed because they never thought to question the idea in the first place, or because they are convinced that the gospels can be read as (more or less) historical accounts and so don't need to be seriously doubted on this point.But why do the overwhelming majority of non-Christian scholars also accept that Jesus existed?Essentially, it's because it's the most parsimonious explanation of the evidence we have.

gospel fragments dating to 50ad-71

If there really was an original form of Christianity that didn't believe this, as all versions of the "Jesus Myth" idea require, then it makes no sense that there is no trace of it.

Such an idea would be a boon to the various Gnostic branches of Christianity, which emphasized his spiritual/mystical aspects and saw him as an emissary from a purely spiritual world who aimed to help us escape the physical dimension.

A totally non-historical, purely mystical Jesus would have suited their purposes perfectly.

Yet they never taught such a Jesus existed - they always depict him as a historical, first century teacher, but argued that he was "pure spirit" and only had the "illusion of flesh". Because they couldn't deny that he had existed as a historical person and there was no prior "mythic Jesus" tradition for them to draw on.

Similarly, the memory of an earlier, original Christianity which didn't believe in a historical Jesus would have been a killer argument for the many Jewish and pagan critics of Christianity.

Jesus Mythicists claim this mythic Jesus Christianity survived well into the second or even third century.

We have orthodox Christian responses to critiques by Jews and pagans from that period, by Justin Martyr, Origen, and Minucius Felix.

They try to confront and answer the arguments their critics make about Jesus - that he was a fool, a magician, a bastard son of a Roman soldier, a fraud etc - but none of these apologetic works so much as hint that anyone ever claimed he never existed.

If a whole branch of Christianity existed that claimed just this, why did it pass totally unnoticed by these critics?

Clearly no such earlier "mythic Jesus" proto-Christianity existed - it is a creation of the modern Jesus Mythicist activists to prop up their theory.

The main reason non-Christian scholars accept that there was a Jewish preacher as the point of origin of the Jesus story is that the stories themselves contain elements that only make sense if they were originally about such a preacher, but which the gospel writers themselves found somewhat awkward.