Aren't infrared waves higher frequency? Why ain't a toaster over faster?

It has to do with the vibrational/translational modes of the water
molecule. Microwaves in the 2GHz range hit it just right.

I believe this is also the reason why some microwaves have a "defrost"
setting that is more than just a low, low, low power setting.

The solid H2O molecule (ice) has different vibrational/translational
modes than liquid water. The microwave must hit it with a slightly
modified frequency microwave in the cavity. Probably throw some sort of
dielectric into the mix.

-Mike K.