There's a history here. Back with Jimmy Carter, he had a primary challenge from Ted Kennedy. Carter won, but only by 51% to 37%. He was already having trouble of course, but it was just one more thing that hurt him and got Reagan in power. We all know how that went. So now the general practice is to clear the field for the incumbent if they choose to run for reelection. Biden hasn't been given any special out.

Trump is of course an odd case since he's not an incumbent, but his grip on the Republican Party gives him a position near to that of an incumbent. He was always by far and away the frontrunner, and every other Republican candidate was for the chattering class.