While I agree, there is the possibility that 1) Biden has deteriorated a lot since last year, 2) some may have tried to talk him out of it, but he / his circle wanted to stay in, or 3) rich donors tried to talk other big name politicians to enter the primary against him and they all thought it was in their best interest to not do so.
I mean, a lot of people are telling him to drop out now and he isn’t doing it. Will also note that it isn’t equivalent to Trump. Trump had some name politicians enter the primary and he won. The big dem names never entered so Biden basically won by default.