Because I've seen some of those models you've made. They look too highly detailed to use in a game, just like the original model for the SWC Rapier is still to highly detailed to use in a game without the framerate going down. (although I was able to import the rapier into Flight Commander and experience little or no lag.)
Also, if you're trying to render too many ships at one time, you'll end up with massive lag, unless you implement some smart code that only renders what the player "sees," or the models that you rendered are models that you aren't using in-game.
Also, will I be able to mod this game when you release it?