He has a record of winning primaries, which every other candidate has not.
He also has a 100% win rate when running against Trump.
Also, presidents don't usually make their own decisions, they have a team of experts for their decisions. No matter if you're 80+ he's demonstrated he can listen and make appropriate decisions.
Also, all the talk about picking a new candidate can be summed up with "of course we should replace Biden with the universally accepted replacement list of 11+ candidates"
So we can't really come to an agreement of who else it should be, and it seems like Biden has all that is needed. It ain't broke, it's just old. And if my 2003 Honda accord is anything to go by, some old things work just fine.
You only need 1/1 win to have a 100% success rate. Statistics can be deceiving.
But since every other candidate has never ran against Trump, that puts them at 0% and anything times 0 is 0, so Biden is infinitely more likely to beat Trump than any other candidate.