From the stand point of technical understanding you have a legitimate question. While the full answer would be volumes, it will be shorter.
!. engines are designed to fit a specific purpose. Cost, Life, weight, fuel consumption, load factor, and reliability. What is being driven and the output rpm and working range, response, are 2nd level but still extremely influential issues to the design.
2. For an aircraft the load factor is quite high. Need for average mission weight, (engine+fuel) is quite important. Life, TBO, relates to total cost of operation.
Even an F1 race car only has a load factor of about 50-80% (monza=83%) WOT for a course and wot time is limited to a 23.5 second burst (Spa straight). An airplane is typically wot from takeoff to descent. Short hops at partial throttle not withstanding.
OK, so what happens? Well, the engine is designed to balance the parameters and the plane has a high emphasis on that mission weight thing. Fuel consumption is big. Initial weight is big.
So, the aero engine is designed around the rpm (Primarily), with prop tip speeds being an issue. Then the bore stroke is limited for piston speeds. This keeps overall friction low and SFC up. Size and temps of components are such to keep operating temps in line, factoring that WOT thing.
Contrast, the motorcycle engine is:
light weight- check
powerful - check
durable - well, maybe 20 hours? (ok, maybe 200)
efficient - not likely, so fuel burn would be an issue.
reliable - maybe, for the 20 hrs and lots of PM.
will need reduction for prop speed.
Auto engines are made to be cheap, and the load factor for a 300 hp street car is probably, hmmm - 7-10%? WOT for, 15-20 seconds, (shifting not counted) and weight, and reliability for flight, . . . . well, it would take a lot of work to get it to work, then, you become developer of and engine designed for a different purpose.
For a fun little, low performance, plane to kick around short distances, and 500 hrs TBO, the auto engines can be just fine. Or high performance for a shorter time.
Reduction drives are a whole, other story, adds weight, must be highly engineered to handle complex loadings. The highly engineered and developed part usually bites low capital investment approach hard. They can work, Rotax shows that. P-51 too. Also affects the engine design , like RPM range.
Better ? too much?