The heat is what kills the organisms and cooks the food. Microwaves can penetrate to a depth of about 2 inches. A food thicker than about 4 inches will heat by microwaves except for the center. Heat here will be transferred by conductance. This is much slower.
Browning requires a higher temperature.
You may have seen very interesting applications of food irradiation discussed on news programs. The press has focused almost entirely on the use of gamma irradiation to sterilize products. One such example is a shelf stable, uncooked steak. In order to do this, the steak must first be placed in a container that will not only keep out microorganisms but will also keep out oxygen. The container is then evacuated and sealed.