As a youngster on a contract at Saints they would:know the expectations, have a home base, be on a great wage comaped to peers outside football, have opportunities to play in Prem- see that it won't stop England managers picking you (AL clearly being closely monitored) and be part of a successful team. They will notice that various "youngsters" have departed to man C etc and are stuck either on the bench or not playing. I reckon most would choose to stay and play until they were mid 20s at least when they can rethink their position, but still be able to command a highly promising fee and be more likely to start matches in the first team of their new club.
But hey, that's what I think!