In short, it's entertaining to watch.
In less short, it represents a competition of physical fitness and co-ordination, which are considered beneficial and attractive traits. While these may not be as important in the developed world of high capital to labour ratios, these were extremely important traits for the rest of human history and evolution. In the past, physical fitness affected your income and success in labour heavy occupations (most of them) and hunting (if we go back further), your health and longevity (still true), your ability to defend yourself and survive violent encounters (far more common in the past), and consequently, physical fitness affected your status and reproductive prospects. Despite that technology is making physical fitness and good co-ordination less important, it is not really surprising that competitions involving these traits would be popular given their historical and prehistorical importance.
I think a better question is why do people view sportstars as role models? It's such a scandal when a sportstar is caught doing something morally questionable, but seriously, why does the ability to hit a small white ball into a little hole have anything to do with expected moral virtue?
"I'm not a role model... Just because I dunk a basketball doesn't mean I should raise your kids. " - Charles Barkley