A talented auto mechanic who learned some basics of how airplanes are different could do 90% of tasks that an airplane would need to stay flying safely. Maybe not stuff like adjusting the balance on control surfaces, but 90% of the rest seems like pretty straightforward stuff. Anyone can work on car, why can't anyone work on a plane?
I've said this before, many times:
An airplane is not a car. There are huge differences in structures, controls, engine duty cycles, ignition and fuel, and many other things. Cars don't have retractable wheels or constant-speed props or prop governors or flight instruments. The exhaust systems in cars aren't glowing red-hot at full throttle, and an exhaust leak in a car isn't usually fatal. Its engine is air-cooled, and the airflow baffling is seriously critical. Airplanes don't have an OBD-II port for the codereader. You actually have to know how to troubleshoot and think, and you'd better know your electrical theory or you'll waste a terrific amount of time and money.
The collection of regulations around aviation are far bigger than around cars and the mechanic has to know a whole lot of them.
This is why the schooling for an A&P is so different. The average car mechanic is dangerous around an airplane because he thinks he already knows most of it. I thought that, too, until I took my AME training (Canada). I found out how much I did not know. And many years later I still have to refer to the manuals frequently. And there are a lot of manuals.
Sure, we here in Canada have the Owner-Maintenance category, but it's restricted to old, obsolete, small designs, and that airplane has to be certified to meet its type design before the OM registration is issued. That means it has to be substantially the same as it was when it left the factory, except for any STCs that have been applied since then. The owner has to keep it that way except for some limited mods he can make in accordance with other certified mods on similar aircraft. In my experience, many of the OM owners end up bringing the airplane to an AME because they start realizing that they don't know nearly enough. They get something messed up so the engine doesn't run right, or they mess up the control rigging and it flies funny, or they get scared that they've done something that weakened the airframe. They don't want to die because of something they didn't know. They don't know how serious corrosion is, and what to do about it.
Here's the study guide for the AME license examinations. We have to get 70% on any exam, knowing the stuff that applies to the category of license we are applying for, and there are four categories. There's also a lot of overlap in those categories. M1 covers aircraft up to 12,566 lb gross weight plus a few old biggies like the DC-3 and some Convairs, as well as helicopter, both piston and turbine. M2 covers the stuff over 12,566 plus turbojets and pressurized aircraft, and heavy helicopters. E covers avionics, and S is sheet metal. The M1 and M2 have to know sheet metal as well, with certain restrictions. Like I said, lots of overlap.
https://tc.canada.ca/sites/default/files/migrated/tp14038e.pdf
It's a long list, and one reason why it takes four years to get the ticket here. Adding ratings takes more education and time.
I've seen airplanes repaired by non-aircraft mechanics. It's scary sometimes.