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.