"Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most effectively on liquid water, and also, but much less so, on fats and sugars.":
https://en.m.wikipedia.org/wiki/Dielectric_heating
In ice the molecules are not free to rotate as they are in liquid water.
That being why they’re bad at defrosting ice unless there’s already some liquid water present, and why they can heat molten glass but not (directly) melt solid glass?
(I’m neither a chemist nor a physicist nor a radio frequency electronics engineer, so I may be way off here).