It's not so simple
1. Why isn't the starter and generator combined to save weight? Ah, dunno, it works, don't mess with it...
They do exist, on jets and turboprops. They are quite pricey... not your $300 starter and your $300 alternator. You are looking at $10k (TBM850)
2. Why are there still carburettors when the rest of the world moved to fuel injection years ago? Ah, dunno, it works, don't mess with it...
You do have the choice of FI so nobody has to fly with a carb anymore. I wouldn't if you paid me for it.
3. Fadec? Ah, dunno, it works, don't mess with it...
FADEC is fairly complex, requires sensors, electronics which have a poor history of reliability in GA (largely because there are very few competent electronics people in GA) and it would not deliver more MPG in cruise than a correctly leaned engine.
4. Will disintegrate if not fed ancient and eco unfriendly Avgas? Ah, dunno, it works, don't mess with it...
Engine efficiency is strongly dependent on the compression ratio...
5. Magnetos that break mid flight when the rest of the world has had electronic ignition that doesn't brake ever for 20 years? Ah, dunno, it works, don't mess with it...
Again, electronics reliability. A poor excuse, sure. But if you had electronic ignition it would need its own alternator. Unlike a car whose engine stops when the battery goes flat.
6. Exhaust Turbo that run the generator instead so you're not siphoning power from the engine? Ah, dunno, it works, don't mess with it...
That has been done, many years ago, but the energy recovery is tiny, for all the hassle you get. Also, making a generator which runs at tens of thousands of RPM is non-trivial. It has been done in recent years, for small turbine engines (look up Bladon Jets for example).