I'm going to assume you mean spectator sports, since there are many well-known reasons for participation sports (including exercise having a strong positive influence on general health, not to mention mental acuity. Did you know that Thomas Jefferson exercised vigorously for 2 hours a day?).

Spectator sports exist because we like to witness competition, and ideally direct, short-term competition between equally matched people. It sets up a fascinating situation - you have two intelligent, capable, experienced players, more or less equally matched, and both have every expectation that they will win a fair fight. Moreover, you agree with both assessments. And yet, they put themselves in a situation where there can only be one winner. Is that not an exciting premise for entertainment?