It is conventional wisdom, but not particularly compelling if you’re actually looking at presidential races. 3 of the last 7 incumbent presidents have lost. But then people will say “the ones who lost weren’t popular”. Except that’s Joe Biden too.
I think the answer to the “why” question is simply that Biden didn’t want to step down, and a serious challenge against an incumbent president is politically terrible. Biden’s defenders are getting unhinged now, after having very obvious issues everyone got to see, imagine if it were coming just because his age was a high number. That’d be a nasty affair. If the incumbent wants to run there’s just no way to challenge them without massively damaging them and causing a rift. Before it was obvious Biden would be basically incapable of winning, it’d make sense to think that would obviously be worse than just having a guy who looks and acts old and might mess up some words.