Because team sports are a great way to satisfy two very important needs: exercise and casual social contact. In addition to making exercise more fun by adding in an element of competition (a common enemy?) team sports, exercising with other people drives me to push myself harder. The structure and objectives (as ridiculous as they may seem in the abstract) of team sports also make this form of exercise more enjoyable than solo sports (which have different redeeming qualities). Perhaps the most overlooked value of team sports is the type of casual social interaction it facilitates. You develop a unique type of relationship with your teammates: one of familiarity and shared experiences/goals, which allows one to communicate feelings that might otherwise be surpressed, but on a level that is surface enough and with a limited frequency and lack of obligation (that might accompany a deep friendship or romance) that avoids the risks of opening up.