Why is mech infantry armoured?

mechanized? maybe

First: Compare, study, and think about all the stats (advantages/disadvantages) between mechanized infantry, motorized infantry, plain infantry, and militia. (This will take many minutes, if you are being serious about it.)

Then: Ask for more information, clarification, or advice.

Yeah, what the heck. They should have put it in the naval category.

They drive in vehicles like APC’s and Halftracks, and use them for cover. They should be a plane unit but that’s just how it is.

you are all wrong, apc is a cover for infantry like a fortress. Mech should be a building category