Taking your comment seriously for the sake of discussion, car engines were designed for different design objectives. There are and were many variables to consider even before avgas became a hot topic. The basic design aspects of a Lyco or Continental horizontally-opposed engine were nailed down over 70 years ago perhaps, but it was an evolution based on aviation requirements through-and-through. Here's a pretty good wiki article on it. I would say form factor and RPM operating ranges are the main differences, but there are many things to consider.

Aircaft engines (wikipedia)

It's not a totally crazy idea however. There have been automobile engines retrofits to modern piston airplanes in a couple of cases, with limited success. The Theilert diesels Diamond used for a few years in the DiamondStar twin was a car engine.