I think that is just the reality of non-mass manufacturing. In the 50's those engines were going into production cars in the tens of thousands, or hundreds of thousands. Now they are produced in numbers of hundreds, maybe thousands, and as such they are hand-built specialty engines, even as primitive as they are.
This is what most people miss. Yes the FAA is slow, certification is costly, and liability messed things up in the 80s -- but you can't get the huge cost reductions we see in the automotive sector without mass-production. Aircraft are essentially hand-made in a labor intensive processes, using individually crafted components.
It's not hard to see what certification costs, take an EAB kit, use all non-certified avionics, and even take an automotive engine and convert it for aircraft use if you want. If you spec that out equivalently* to a certified aircraft, taking labor costs into account (you don't get to count the 1500 hours in your garage as free), it's less than comparable certified aircraft, but not orders of magnitude less.
(By equivalent, I mean safety and redundancy -- ifr capable. Not talking about making my own AP with cheap servos and a raspberrypi and navigating with a handheld gps)
I think the mooney bravo had a porsche engine at some point? But otherwise you are correct. It wouldn't make any sense to use engines in both cars and planes because they are such massively different requirements: One needs extreme reliability at basically one RPM and power output and has things like double spark plugs for robustness, the other wants to keep costs down and reach higher RPMs
