Here is a site that gives a pretty good description of how a microwave oven works, and then compares it to a convection oven (which is a regular oven with a fan that blows the heated air around)
https://homequicks.com/convection-oven-vs-microwave
From the above site “in case of a microwave oven, food is cooked by passing non-ionized microwave radiation through the food. The water and fat in the food absorb this radiation by dielectric heating. The heat is then distributed throughout.” So although the inside of the food gets heated (from the inside out) it is more like steaming. The air inside the oven stays at room temperature, and cannot brown the food.
A standard oven heats the whole inside of the oven, baking the food, which also browns the food, and keeps it dryer.
Luckily, newer technology in packing frozen food, as well as different methods of creating the frozen food, make it possible to get crispy crusts on some items, like the pot pie I mentioned above, but I think the drier cooking method, of a comventional oven is what makes it brown, and have a “baked” texture, and therefore taste better, but I am perfectly happy with my veggie microwaved pot pies.