I think they are under more of a load a cruising speed than you'd expect. At speed both the car and plane are primarily overcoming wind resistance, which is more about speed and elevation than anything. If I'm understanding right the plane would be going more than highway speed within a few thousand feet of the ground for most of it's (flying) life so it's probably putting more stress on the engine than a car on the highway would.

More importantly: if your car engine dies you gradually slow down and pull over. If your plane engine dies calmly pulling over isn't an option. It's much easier to justify spending the money ensuring a plane's engine is reliable.

They're generally outputting up to around 75% of their rated horsepower at cruise.

I'd say that's closer to going 110km/h or 65mph in 2nd or 3rd gear for most cars.

For example, a typical performance cruise power setting in a Cessna 172SP might be 70% rated power (180 BHP at sea level), which is way, way, way more than you'd ever use for a sustained period in your car.

Highway cruising might take 20-40 HP depending on your vehicle, which is probably more like 10% of rated power. 70% of rated power in a typical American car is probably like 120 MPH.

To put the power-over-overhaul-run into a more comparable perspective, I think it's helpful to compare the gallons of fuel consumed [as a proxy for the amount of energy/work output]. A small aircraft engine like in a 172 might be 10 gallons per hour, or 18K gallons of flight time over an engine run.

18K gallons will take a typical road car around 500K miles, so an airplane engine getting "only 1800 hours" of flight time likely outperforms the typical auto engine.