You're making a category error. Biden is the incumbent. The Party always runs the incumbent. Is that how it should be? We can debate that, but that's how it is.
There's no law that they must run the incumbent; it's just tradition (and a tradition with some reasonable underpinning... "Why should we listen to you if you don't even back the guy you sold us last time?"). But many voters are exhausted of tradition and fearful of where it may lead.