I don’t believe the dems choose as much as promote. I believe Biden had earned the slot with obedience and time-in-service but they didn’t really want him. They knew Trump was a lock for a second term and gave Joe the chance to run but everything went to hell with Covid and gave the dems the perfect opportunity to flood the country with mail-in ballots. Pretty sure they regret having Biden on the ticket but his deep state handlers have really taken advantage of their opportunity.
any evidence for these beliefs?
Just the same evidence you have. The DNC is nothing if not deeply organized, far more than the RNC. Politics is a profession and they prefer to promote from within. DJT was an outsider and that went against their corporate policy. Just look how hard they’re working to make sure it doesn’t happen again.