Something to remember is that general aviation planes don't typically use modern complicated engine designs. Complicated engine designs are one more way for something to go wrong and planes often fly in very different conditions than cars. Moreover, 40 year old planes are flying 40 year old engines, or at least the designs are that old if the engine has been replaced. It's not legal to just swap a new engine type into an old plane.

GA planes also have tight weight tolerances. They just aren't designed to carry a lot of extra weight and so an engine with lots of extra stuff could easily use up a lot of useful capacity, especially on a four seat plane which is really "two seats and luggage" or "four seats and no luggage".

One of the common questions people have is why planes don't use automobile engines since they are commonly mass produced. The issue becomes that you need a transmission and the extra weight of the transmission makes it difficult to convince anyone to even try getting it certified. I only know of one attempt at it.

Using the Cessna 172 as an example since it's so common, the Continental O-300 engine used for many years is a 6 cyl 300 CI 145 HP engine producing about 230 ft-lbs of torque at 2200 RPM. Note: plane engines can't rotate fast because the tips of the propeller would break the sound barrier and the shockwaves would cause issues (and by issues I mean death and destruction). How many 5 liter/300 CI 6 cyl engines do you know of in cars and how many produce much more torque than HP?