The server would have to keep track of each teams scrimmage schedule for too long. Also if a team decides to schedule a hundred scimmages in advance and then the quits the game it can be frustrating if he doesn't set a lineup before he goes bot.
There is no real advantage to scheduling scrimmages in the future as scheduling a scrimmage doesn't take long, and you won't know what your lineup is for the scrimmage anyway because injuries, foul outs and minute distribution determines the lineup for the scrimmage.