Because he's an idiot, or at least has no political sense.
Among the list of plausible reasons...
- Trump wants to do it because he wants to show he can beat them on their own terms and that he has no fear. He knows they will say he is afraid to debate if he holds out.
- Sheer vengeance and competitiveness.
- Maybe he’s hoping CNN overplays their hand. That would help him.
I believe Trump will beat Biden to a pulp.