That's the power of positive and negative reinforcement. They want to shape the behavior of players. They want players logging into the game multiple times a day, every single day.
So they rate limit the player's game with "stamina" or whatever euphemism for a timer they come up with. Player was having fun playing the RPG with friends, only to run out of "stamina"! The choice is to stop and wait for the timer to reset or pay money to reset it. Then the player starts waking up at 3 in the morning because that's when the timer resets.
The simple timer reinforces a schedule and creates the behavior the game developer wants players to have.