Too many West Wing watchers talking about empty chair debates!
May would absolutely steamroll through Corbyn in a debate anyway. Labour should be delighted to get away from the prospect.
Why wouldn't she then?!
Such debates always favour the minor players and not incumbents and allow the others to gang up on the incumbent to show their own views.
May has nothing to gain from a tv debate and plenty to lose. She is not a natural performer and not as vain as Blair and Cameron who were style without substance and natural performers. She knows she would leave some hostages to fortune. AT PMQs she shows that she has some grasp of the facts but just repeats prepared lines.
On top of all of that she doesn't need to do the tv debate while the others do need it for exposure.
In addition she does not want to give Corbyn a sympathy vote as she treats him in a derisory fashion.