Who should Biden worry about more? Getting the left wing of his party to nominate him, or getting the general electorate to the polls in November? It's a no brainer calculation.
It's a proven strategy for the Democrats. That's how they got Trump, and that got them tons of funding and dropped the bar all the way to the floor. Why would they give up now?
This is the disconnect between the DNC and normal Democrats. Everyone who's not in party leadership can see how absolutely idiotic this approach is, and the DNC is so far up their own asses they are incapable of recognizing it.