I think that for most voters, the "safe" option is to just stick with the incumbent - who has proven they can win - rather than go with some unproven second option that *might* win but also could just as easily crash and burn. Even if Biden isn't a very strong candidate, replacing him with someone else has the potential to make things worse.
As to why Biden had no credible primary challenger (Dean Philips and co were not credible in my mind), primary challengers against sitting presidents seem to A) have little chance of beating the president and B) weaken the president and cause them to lose. You have Bush with Buchanan in '92, Carter with Ted Kennedy in '80, and Ford with Reagan in '76. Those are the most significant primary challenges to a president that most Americans can remember, and all ended up with the incumbent losing re-election in the general (go back a bit further to '68 and you have Johnson being chased out of the race by Eugene McCarthy and the eventual winner, Humphrey, losing to Nixon). So primary challengers to presidents do not have a great track record, and nobody wants to be the guy responsible for Trump coming back (this is another part of the equation - Democrats fear and hate Trump more than anything).
The real answer to the bolded was already brought up a few posts back, though. Primary challenges don't weaken the sitting president. The sitting president gets challenges because they're already weak.
