I mean, there's being cheap and then there's getting a new aviation engine FAA certified. Those aren't exactly the same thing and Bob the dentist who likes to take his Cessna 172 up on weekends can't simply approve a new engine.

This is why buying a new spec Cessna 172 from the factory floor today will still come with the good old Lycoming O-360 engine, introduced in 1955.

Cessna sells ~250 172s per year, nowhere near enough to justify new engine approval.