I don’t think he has any reason to debate anyone right now. He’s the President, and the Republicans haven’t even nominated anyone yet. Trump is the front-runner, but several things could prevent him from accepting the nomination.

He could be disqualified under the amendment, he could be incarcerated for one of his many crimes, he could be forced to declare bankruptcy as a result of judgements against him, or he could have a health problem that prevents him from continuing his campaign.

Biden won’t win anything in a debate with Trump now, and he can lose a lot. Trump won’t have any shame about levelling accusations, and could easily just make up some bullshit on the spot. Trump supporters will take it as gospel, and now Biden has to spend political and financial capital disproving some absurd allegations.

Biden is better off waiting for the actual debates, and letting Haley take the pot shots at Trump.