Because microwaves don't heat matter. They heat* H2O molecules. This one weird fact is responsible for all of the weird differences between how they cook and how other more classic cooking methods work.

We're taught that heating has three styles: convection, conduction, radiation. But AFAIK, microwaving is a fourth and distinct style.

*: Even more specifically, they add rotational momentum to these molecules, which is not the same as heat, but gradually turns into heat (which is translational momentum) as they knock around. This, in addition to the fact that only the water is being heated, and that the microwave waves touch the food in an uneven pattern even if mitigated by a rotating platter, is why stirring or waiting or using "low power" (dithered) is an important part of microwave recipes, as well as why high moisture foods or intentional steaming works so much better in it