First of all, there is a whole damn FAQ section dedicated specifically to this exact question.
That said, there is a vast difference between Jesus the son of God walking the earth existing and Jesus the Gallilean Jew and minor apocalyptic prophet existing. The former is very much not the mainstream view, the latter is. Now, again, I recommend you go read the FAQ but broadly speaking the facts most historians agree on are very basically this:
Someone called Jesus was baptized by John the Baptist and crucified, he probably had a few disciples, there was probably some kind of controversy at the temple and his disciples probably continued his work after his death, some of them suffered some form of persecution because of it.
Now, there are two possible explanations for how this narrative became accepted.
The person really existed.
There was a vast and complex conspiracy by a diverse group of people who seemingly stood to gain nothing but still were willing to risk persecution just to spread this narrative. Despite endless scholary research they were so skilled that no evidence of that conspiracy ever surfaced.
One of these solutions seems vastly simpler than the other. Apply Occam's Razor liberally and you'll see why most historians tend to favour one of these theories over the other.