Debates aren’t part of the election process, they’re part of the entertainment process. And if Biden doesn’t agree to do the age-old Three Debate monkey dance, the Undecided Voter will grow angry and support the Fun Candidate instead.
Or, at least, that’s a theory. There’s another theory that debates are dumb nerd shit that nobody who hasn’t already made up their minds watch anymore. And - much as a bunch of Senators and Governors have refused to debate during safe election years - Biden’s refusal to meet Trump on national TV will actually make him Cool and Based.