“I also blame airframe manufacturers. Why do they NEED a separate variant? The Piper Arrow has an IO-360. Why the bleep didn’t Mooney choose that engine when designing the E, F or J models?”
I can’t speak to all of the differences, but in the case of the IO-360 I know of the reason for at least ONE difference. The Cardinal RG and some Mooneys both use a near-identical engine, the IO-360AxB6. On the Cardinal RG the ‘x’ is a ‘1’, and on the Mooney it’s a ‘3’. That digit refers to the location of the prop-indexing bushing on the crankshaft flange. Not a different crankshaft, just a different location for that pressed-in bushing. Which, in turn, changes how the prop is ‘clocked’ to the crankshaft.
Which apparently is a significant enough difference. Mooney and Cessna must’ve found during development of their retracs that some prop positions undesireable vibrations, and had the crankshaft indexed accordingly. Same engine, yes, but mounted in a different airframe produces a different result.