“But no one knows why so many runners get hurt, although a number of theories have been advanced, including the possibility that hard asphalt roads, lousy Western diets, too many miles, too few miles or high-tech running shoes cause or contribute to the problem.”

Phys Ed: Does Foot Form Explain Running Injuries? - NYTimes.com

I’ve come to the conclusion that it’s because in most situations. running is simply not natural.