The goal of elite university admissions is not to maximize academic success, it's to maximize career success and prestige. Organized athletics at the highest level require high levels of teamwork and grit which translate very well to the work force. Grit and teamwork might actually be the most important attributes for having a successful career so it really surprise me when people dont understand why athletes get some preference in admissions.
Aside from scholarships for sufficiently talented, it certain places it sometimes viewed as demonstrating leadership and/or social skills. Especially if you have some type of leadership role (captain, etc.). And especially military academies.