Per my previous (two posts back) reply, device limitations in regard to switching speed is the main reason. To get 99% modulation depth, the switching devices would have to be capable of switching at 100X carrier frequency, hence at 7 MHz, the devices would have to produce something resembling a square wave at 700 MHz.
There are ways around this problem. Harris series combines a bunch (48?) PA modules, arranged in binary order, big steps, and little steps. Their drive signals are switched on and off according to how much instantaneous power is required for the envelope power as dictated by the incoming audio. Nautel at this point is conventional PWM with DSP based AM/AM and AM/PM correction, similar to the smaller DAX series transmitters from Harris/Gates/Who ever they are this week.
My attitude is, if it were possible and financially viable, it would already be in a commercial product.
+-RH