Isn't the obvious reason because incumbents tend to have an advantage? And isn't the other obvious reason because he's already beaten Trump before?

Of course, there are other Democratic candidates who would probably beat Trump. And perhaps many of them could beat him much more decidedly.

But if you have something that got the job done before, it's always a risk to try a different tactic, even if you know there would be some options that would theoretically be better - because you don't necessarily know WHICH options those are.

Certainly, Biden has his shortcomings. But most of his shortcomings - including his age, and tepid support at best from pretty much anyone - were already there 4 years ago. So I think the more interesting question is, why was he allowed to win the Democratic nomination 4 years ago?

But at this point, honestly, it seems crazy that people are still questioning why he's running again. I'm not even saying that it's the best decision for him to run again. But the reasons why it MIGHT be a good decision are obvious, and don't have anything to do with him having delusions of grandeur, or anything like that.