A cynic might also suspect that the Dems just might have a more appealing candidate in mind, and will wait until the time at which Joe can pretend a reason to exit the race, with little time for the Dem voters to have input in a (haha) "democratic" primary.
Just my guesses, the Dems and I don't talk.
the party will choose the candidate
you know
to strenghten democracy
It could easily happen that the Democrat powers that be switch out Joe Biden for someone else.
There is a big group of so-called super delegates at the convention who are not officially pledged to any candidate.
And then, if we go through the upcoming primary season where Biden has little or no opposition, and wins the bulk of delegates, and after all of those primary elections are held Joe drops out, then all of his pledged delegates at the convention would become free agents.
We could find ourselves next May or June where Biden is the presumptive nominee based on delegates won during the primary season, but then at that point Joe decides not to run for reelection. Then the upcoming Democrat convention would have no presumptive nominee. It could be a wild convention if this happens.
If Joe drops out before all these primaries start , up then we could see if you have a presumptive nominee based on the primary results.
But if Joe exits after all those primaries are held and he has all those pledge delegates that suddenly are free to vote for whomever,, all bets are off.