But if they stayed the same, nobody would fire their trainer...you buy one and that's it. Taking care of the staff is one of the things you have to manage well and personally I don't care if staff salary stays the same or raises each month, as long as it's the same for every user.
Seasonly would cause a run for new staff at the start of each new season and the prices would skyrocket.
And this would also be easily avoidable by firing your trainer at end of the season, train FT or ST for two weeks and wait for buying frenzy to finish, then buy a new trainer.
Sometimes from a point of view of game mechanism, we have to stay away of the reality.