Auto-mixture
 

Regarding piston engines, why aren't there many engines designed with auto-mixture, instead of having the pilot meddle with the mixture lever manually?

My 1953 Commander has pressure carbs and mixes it all automatically - never have to touch them. It auto leans with barometric pressure. Don't know why it never became more widespread.

It's a long answer but basically aero engines run mostly at a constant power setting so the mixture adjustment is a non issue and it is simple and reliable.

The engines are designed to be lightweight and this means they are unable to generate their max rated power at the most efficient fuel/air ratio (stochiometric combustion; roughly at peak EGT) without the CHT going excessively high, so the mixture adjustment is provided to enable the selection of a very rich mixture during poor-cooling high-power regimes (climbing) and this reduces the CHT. Some notes here.

Everybody with any sort of half decent education who gets into aviation wonders about the antiquated engine controls but actually they work fine.