So modern piston aircraft have things like fuel injection. ECUs no. It's mainly because there hasn't been a reason to innovate. Piper and cessna (the two largest general aviation manufacturers) haven't had a reason to innovate. The engine suppliers of which there are 2 main ones, haven't had a reason to innovate on anything. Diamond and cirrus are changing this. Diamond is using modified Mercedes diesel engines so they burn JETA (jet fuel is similar in a lot of ways to diesel fuel) their planes also are FADEC controlled. Meaning they do have an ECU that manages the engine. So no controlling fuel flow with a mixture lever, no adjusting the propeller with a lever. The system handles all of that. Add in that it's very expensive to certify a new engine and with little demand for anything new means manufacturers have been able to roll out the same airframe and construction for 50-60 years.

Long story short. Expensive to not on design and engine plus certify it and all its components for flight with little demand for innovation means companies won't bother.

That's not really the case. You will find engines that are full of electronics and pretty much comparable to modern car engines.

However! Keep in mind that certifying a new aircraft engine is an order or two of magnitude more complex and expensive process than anything that gets into a car. You want to change the type of spark plugs? Change to an ignition system? Well, tough luck! Needs to be recertified.

There is also the fact that unlike a car engine, where if you get an electronic failure your check engine light comes on and you call a tow truck, in an airplane if something dies, it is a full blown possibly life-or-death emergency.

So you do not want to stuff your plane with a ton of electronic gadgets that could fail. The simpler the better in this case. Or you must provide redundancy, which is very expensive compared to e.g. a pair of magnetos for ignition. The other issue is weight, especially for ultralights but even for bigger aircraft.

The more complex electronics you add to the engine, the more it will cost to have it certified and even more to maintain. Keep in mind that a modern Cessna 182 costs upwards of $600 000 new today, the engine and related avionics being a large part of it.

You can't compare this at all with car engines. It is a completely different regulatory and reliability ball game.

There are plenty of modern electronically controlled (FADEC - that's what you know as ECU from car engines) and fuel injected aviation engines around.

However, typically not in your flight school's 40 years old Cessna 152/172 but in bizjets, larger helicopters and high end GA planes, like Cirrus SR22, DA-62, for example - which has two diesel (!) engines. Lycoming produces multiple engine types that are fully electronically controlled, Continental does as well, Rotax has several fuel injected engines, etc.