It’s all a question of frequency. Traditionally, humankind has used waves of infrared frequency to cook food, either directly as in grilling or broiling, or indirectly, where the fire heats the pot and the pot, by direct contact, heats the food. Infrared frequency (what we normally refer to as 'heat') cooks the food by shaking its molecules in a more or less aggressive way, according to the degree of 'heat' applied.

Microwave ovens are different. Their relatively short, violent bursts of vibration in the longer microwave frequency band can only heat certain chemicals, principally the oxygen in water, but also the oxygen in amino acids, lipids and proteins. Whereas infrared waves induce a shaking motion, the alternating microwave electric current generated by the magnetron in every microwave oven forces the food molecules to rotate (reverse polarity) at its microwave frequency (1-100 billion times a second). The friction from this violent, thrashing motion tears at the food, vitamin and enzyme molecules, destroying, for instance, their cells' walls, while heating them savagely,[1] and changing their shape.[2]

No indirect heating via pots or plates is possible because hard materials resist rotation of their molecules. The pot or plate is only heated from direct contact with the heated microwaved food.