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.