Because leveling complicates things too much for people like me. I used to like complicated games. But I've grown ultra tired of hard learning curves. I had having to make 'builds' 'just right.' When I come home I wanna have fun, not do more work. Not that leveling, configuring and grinding isn't necessarily unfun, but for me and people who don't have enormous amounts of time to spend it's not good. This would be game is only single player or co-op.
Going back to what I said. You could command Starfleet yourself, give orders and whatnot. A battle could be completely fought by the ai following your orders to fight. You could also be an Admiral which gives tactical orders during a battle, or just a high up guy who tells the fleet to engage and an ai admiral does that instead. (I am a huge NPC enthusiast.)