Why does heating something (like pizza) in an oven make it crisp but heating it in the microwave makes it all soggy?
I am not sure that this question is answerable at all - it seems to have a false premise. In my experience, heating anything made with dough in the microwave makes it dry and hard as a stone, not soggy. (and we have already answered why this happens).– rumtscho ♦Apr 29, 2012 at 10:39
2@rumtscho You can definitely make soggy pizza (especially any of the American styles with plenty of toppings) by reheating it in the microwave. There's plenty of water from other ingredients. If it were thin crust and minimal toppings, sure, it'd get tough, especially after cooling, but the question is fine.– Cascabel ♦Apr 29, 2012 at 17:21
Possible duplicate of Why do microwave ovens make bread rubbery?– GeorgeMar 21, 2019 at 14:06
Microwaves specifically heat water molecules in the food. This turns them to steam, and because the air in the microwave is actually cool, the steam then condenses. There is often not proper air circulation to move the steam away from the food. Often times the outside edges of the food will not be soggy, but rather burnt, because they receive more energy and the water can totally vaporize.
For better results when reheating food, do not reheat your food on "high" or the default power. Reheat it at a lower setting for longer time. For pizza specifically, I heat it in the microwave only to room temperature and then finish it in the toaster oven or in a pan.