For some reason, runners are often an object of unique curiosity to non-runners.
Some people are sincere when they ask the question.
However, my unscientific polling over the past 3 decades clearly indicates that these folks are in the minority. People who grew up in the late-60's seem to think we do it for the endorphin rush. Frankly, the answers some runners give to the big question are quite boring. In a way, the interrogators get what they deserve. It sort of fulfills the old Chinese proverb which cautions us to be careful of what we ask for as it just might be granted.