Why is putting an egg in the microwave not recommended?
This probably refers to whole eggs mostly...

A microwave can boil water very rapidly, and a tight but fragile container like a whole egg will violently rupture if such rapid boiling happens inside it, because the overpressure inside it is already significant when the shell finally breaks.

Here is a video showing an experiment with ca. 180 eggs in a microwave oven, which after a minute or two rupture simultaneously and with enough force to blow the oven open, with debris being propelled several meters.