Automotive engines are designed to produce power at a wide range of operating speeds, at the detriment of peak power output and peak efficiency. Aircraft engines can take advantage of the fact that they will be operating at a single RPM range for most of their life, which allows you to optimize every single aspect of the system for that speed. Aircraft engines also have much, much higher reliability, lifecycle, and quality control requirements than automotive engines.

For these reasons and a few others, you would basically end up redoing most of the engineering work on an automotive engine to repurpose it for aerospace applications, thus negating the savings of reusing the existing design.

But you can do it the other way around, resulting in the M4 Sherman. :-)

Reliability would be my first concern. Power to weight is likely an issue as well. Here is some more info I found:

https://www.flyingmag.com/aircraft/do-car-engines-make-good-...

Although probably not the majority, many experimental aircraft do use car engines. I'm building a Zenith 750 kit aircraft and installing an engine from a company called Aeromomentum who convert Suzuki engines for use in light aircraft. The model I have installed is a 117HP engine originally designed for use as boat outboard motor. A sibling comment mentions that car engines aren't designed to operate continuously at high power. Outboards are. And there are benefits to using highly developed and tested engines vs traditional aircraft engines - efficiency being the most obvious.

Read the many reasons below, but one big one not named is that auto engines are not designed to be operated at near peak power constantly.

Auto engines are severely derated for use in airplanes, and then all of the advantage is gone.

Some do, but not many

http://www.epi-eng.com/aircraft_engine_products/big-block_62...

They do, though it requires some conversion to meet reliability requirements and to optimize for cruise. Over the years the Subaru boxer engines have been a popular starting point.

Efficiency at sea level does not equal efficiency at altitude. That said, there are some very impressive experimentals that do use auto engines.