Aircraft mechanics do not *just* work on the powerplant (engine). Most will get their A&P license to work on the airframe (outside mainly) too.
Things like preventing corrosion like rust, and tightening rivets to make sure the aircraft does not come apart, and changing oil. That is just to name a few of the things A&P mechanics do.
Or, if you want, you can get a certificate in Airframe instead of Powerplant; we don't just work on engines.