I know why this is! It's multifactorial but the biggest factor is impurities in the plate, especially ceramic plates, that are polar and/or metallic and DO interact with microwaves, absorbing some energy. Since the specific heat (the amount of energy required to raise the temperature of s substance) of the plate material is typically much lower than food, which contains water (which has a very high specific heat), it'll heat up to a pretty high temperature despite not absorbing as much energy. It remains hot as long as it does as it doesn't contain much or any water, unlike your food, which also provides evaporative cooling.

Melamine plates aren't microwave safe.