[mkarr]

A large part of these technologies making their way into consumer automobile engines was advances in material science, lubrication, manufacturing, etc, that allowed for them to exist economically and reliably. Those WW2 era piston engines were horrifically expensive, and required dozens to hundreds of hours of maintenance per hour of flight time.

[Gravityloss]

Sleeve valves could enable higher compression ratios without detonation, for higher efficiency? I guess there is the NOx to consider nowadays though.

[Blackthorn]

They were also ridiculously unreliable and had the tendency to just catch on fire. Acceptable losses in wartime. Not so much elsewhere.