It is because of biological reason, not because of physical reason.
The optical cells in the eyes of human retina are sensitive only to electromagentic waves (電磁波) of wavelengths ranging from around 400 nm to 700 nm. This is the reason why we can "see" eletromagentic radiation within such wavelengths.
Microwaves are electromagentic waves with wavelengths from about 300 MHz to 30 GHz, which is outside the response capability of the optical cells in our eyes. That's why we cannot "see" microwave radiation.