If he bowed out and the Dems nominated a halfway decent candidate (which they likely wouldn't do, but that's a different subject), they'd demolish Trump. He'd lose so badly he couldn't even pretend it was fraudulent (though of course he'd claim that anyway, since he has the emotional maturity of a spoiled five-year-old). The race would instantly go from a terrifying risk to a complete rout.
But between Biden's ego and the DNC's determination to stick with a wholly-owned establishment neoliberal hack at all costs - even if it means losing - I expected that they wouldn't take this golden opportunity.