a 10 km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants. For comparison, a solar PV farm of similar size might easily exceed 10,000 megawatts (rounded) at best conditions during daytime.
So, if a solar array, of the same size as a receiver antenna for microwaves, is over 10 times more efficient than the microwaves, why bother?