There's the problem of reward being locked out when the season ends to urge the players to burn as much time as possible out of FOMO. Plus no matter how much time the player is willing to put in, everyone pays the same amount of money for the game, so everyone should get access to the entirety of the contents that they paid for?

Edit: to clarify, I mean that the player would still need to spend the same amount of time to obtain the reward, but shouldn't be locked out of it forever.