The way this game functions, a user cannot keep a lot of his players, unless he wants to have a significant disadvantages. He has to sell most of them every season, unless you are one of those users that can earn 50-100 green packs daily from 'free' offers. And even in this case, the user still has to sell most players after 2-4 seasons. You cannot do it the real life way, that is buying a youngster, develop him and keep him until he reaches the 30 year old mark or more.
This is one of the business strategies in this game. A user has to refresh his squad or make use of a lot of green packs - in other words buy tokens and/or complete offers.
In doing the training and selling, users are doing a lot work indirectly for Nordeus. As a user, your benefit for the effort put in finding the player, training for special ability, extra positions and stars is temporary (usually 2-3 seasons). Sometimes, the player only delivers in one season, and is poor the next season which is another disadvantage.