Apparently the only way the candidates will agree to do it is if the format is so stilted that there's no chance of anyone learning anything or seeing the candidates get challenged on anything. It's basically just a taking-in-turns version of a campaign commercial.
What, indeed, is the point. Like a lot of American politics, the whole "debate" survives as a pointless vestige of a thing (now long forgotten) that was useful and productive in its original form, but now is mutated to a useless and unrecognizable monstrosity, which you have to pretend is super serious and important if you want to be able to be on TV.