Even though it's important to remember that there's no actual evidence I think those saying there was a guy it was based on are probably right. It's how things mostly work isn't it.
Not the miracles of course. If the dead had really been walking around the town when he died ALL the historians would have mentioned it.
Also because everyone is so used to the story now it seems fine, but it's not the greatest story ever told is it. He preaches a bit. Causes a riot or two, smashes up some stalls in a temple/market and eventually gets arrested, tried and executed.
Okay it worked out and now he is a household name (or a name possibly similar to his name is a household name), but surely 'preached until he was 92, converting tens of thousands and then crucified' would have been a better career/story. Especially if that meant half a century of his thoughts in writing left behind. I bet all Christians would prefer that.
What is there doesn't all paint him in a good light either. I mean sending the disciples to fetch a donkey so it would look like he was fulfilling the prophecy. (Matthew 21).
I wonder if they all did that? Maybe there were donkey traders on all major routes into Jerusalem taking advantage of the need. (with a sideline in gourds and sandals perhaps)