Gears. The car's transmission alone has probably 30 meshed together on its own, not to mention gears are in everything. Bearings too. Is a sphere a wheel? Casters, slide rails with wheel runners, washers, and conveyor belts feel like they deserve a spot as they are round locomotive devices.

Doors also are very rare in nature, but a rock pretty regularly forms the right shape for a wheel.

I might be missing the forest for something that looks like trees, but it feels like there are overwhelmingly more wheels than doors.