Well rubicon789 i used to think that also. but the reality of it is it just does not hold up. the rotary installs are starting to really prove themselves with many 1000 hour + birds flyin. the subaroos are even more proven.
the problem with auto conversions is weight and systems. most of the engines weigh way too much, so do not make a good conversion candidate and on the ones that do, it is hard to get all of the systems right.
in the 30's, 40's and 50's aviation led in making improvements fast and furious. unfortunately, those days are long gone and it is basically up to people in thier garages to improve the state of the art.
the aircraft engine mfgers have such a small market that they cannot make any real headway in engine development. heck, in the auto world the annual production run for an aircraft engine would not even be big enough for a prototype run.
Now, since you can rebuild a rotary for about $1,000, if they continue to prove themselves as they are, you will also become a convert. the subes are also inexpensive to rebuild and have an excellent track record.
thielert took a mercedes auto diesel and converted it to aviation diesel and under powered for my bird - it is doing very well and is available in many certificated planes.
on the surface - you would seem to be right, but, MANY are proving that the auto engines are more than robust enough.