Because their mindset is let an injured player take up a spot on the roster until it's painfully obvious that they need that spot, and don't have any real depth, so keep the player sitting there doing nothing on that off chance the magic health fairy makes the boo boos all better overnight rather than use the spot for a player that might contribute.
Of course it could also be they don't think it's bad enough for 15 days off and will get better sooner than later. But we've seen how that hasn't worked well for this club before.