That's some of it too, but in a way legs aren't all that different from wheels in how they work. Most of it is the gearing. Like there's a gear on every mountain bike that will make your feet go down at the same intervals as walking. That sucks on flat terrain. Legs are also not perfectly designed to push pedals, especially for people who don't bike often.

I believe the "wheels on the bus go round and round" :-)