Reason 1 Fallback If an athlete does end up becoming pro, they can get injured and end their sports career. So, it is good to have some education to get a backup job. You can be a coach, have a business finance, or have a health/physical education as a big backup help.
Reason 2 Maturity: These athletes may not be mature enough. If they're 17/18 and they've won a game, these kids are going to go back to the hotel and play games rather than going with the pros to the club and celebrating. No one will understand why they're doing that. What if they lost a game? Their reaction might be out of control and the audience will not like it. Athletes in their 20s will just shake it off with a shrug and think, "We will do better next time."