The God of Convenience

[Mudhammam]

Why do so many atheists believe in the existence of one and not the other?

Because this:

I believe that Plato and Socrates existed based on the same type of evidence we have for the existence of Jesus, except that we have more and newer evidence for Jesus.

...Is entirely incorrect.

You accept the evidence for Plato and Socrates, but not for Jesus.

Right. Because one (Plato---Socrates is more disputed) is substantiated by other evidence, such as his contemporaries, his critics, other historians, all with reputations, archeological excavations, and analogy with events we witness throughout the world, today and in times past. Jesus Christ is not. Jesus was a figure with very little historical importance until the author of Mark's Gospel placed Christian theology in a specific time and location, probably in response to the Docetics, and also because that's how revered teachings of figureheads, mythical or not, were recorded.

You do realize it makes zero difference whether or not Plato was an actual person, though, right? When Christians can say the same about Jesus, their arguments for historicity will start to look more credible.