A car engine rated for 80 HP will spend most of its time producing 30-40 HP. An aircraft engine rated for 80 HP will spend most of its time producing 55-60 HP, and thus needs to be a good deal sturdier.
From the engine's point of view, an aircraft as a load is more comparable to static use, like in a compressor or generator, than a road vehicle.
That said, in the world of non-certified aviation there are a good many car engine conversions. The low cost of acquisition makes up for the loss in power/weight ratio and reliability. Subaru have already been mentioned but that's mostly a North American story.
For just one European example, the Belgian Calypso microlight can use engines from a Citroen Vista, but also from a BMW motorbike.