I think it's as much to do with the flatness as it is the sheer hardness. If interior floors were cobbled and at angles, our feet would be required to work a lot more and our stance would change to fit. We'd also probably wear thinner and more flexibly-soled shoes.

As it is, a constant flat hardess, coupled with shoes, only serves to constrain the foot and work singular positions.

I know if I'm on foot standing all day in the workshop (concrete floor) my feet hurt a lot more than if I'm out walking on the street for the same sort of duration.