#1 Because he obviously believes he can win. Do you have any evidence that he knows he can't, or are you just being disingenuous again?
#2 Sanders supported postponing the primary until it was safe to vote again, that would have given the states enough time to develop remote voting or at least safer protocols if in-person couldn't be avoided. If Sanders had no chance to win, what's the difference if the primary is in March or June?
#3 There are down-ballot races that need to be determined. Sanders dropping out doesn't stop the primaries from occurring.
But hey, do anything to excuse Biden's terrible leadership and decision making, right? That's quite the theme these days.
The fact is, I find myself grinding my teeth occasionally at the way some centrists gleefully spit hatred at the left. It seems to me that's about self-interest. The affluent and comfortable don't want the boat to be rocked too much. And at some point, if the situation changes, I believe some of those people will openly align with the right.
But...right now, now more than ever, the enemy is Trump and what he represents. If it wasn't already apparent, the pandemic makes it quite clear he and his hard-core supporters are going to get a lot of people killed. I do not believe that Biden, for all his many weaknesses, would have behaved so destructively in the face of a crisis like this. Trump is a dangerously-flawed personality, he's potentially the hand-maiden of the grim reaper. His racism and climate-change-denialism alone are potentially disastrous.
And the primary is over. Biden won not because of a fix by the relatively small number of very affluent highly-educated liberals, but because ordinary working people who voted were not persuaded of the need for something less conservative (I was going to say 'more radical', but I don't really see Sanders as 'radical' - he would be mainstream left in many other countries, even seen as right-wing by some of the left here).
I would have preferred people to have voted otherwise, but people are what they are, and you can't "dissolve the people and elect another".
Get Biden elected and try to hold him to account and try to make him actually do the right thing. The alternative is too bleak.