1) maybe Biden is the only Dem candidate with a broad enough appeal in swing states (scary, but maybe true)
2) Dem's are confident they can push Biden through to another victory or
3) That they can do a bait and switch later in the race and still win
4) Biden might be a unique "compromise" candidate that was essential to the 2020 steal (obviously not because of his appeal, but because all of the critical Dem factions could be brought together to support and cheat for a puppet president)
Of those choices I would be most inclined to believe that #3 is most likely afoot here. Biden AND Harris bow out at the last moment and the dems get to appoint anyone they want to run in his place. There’s lots of communists for the dems to pick from too if that’s the case.