Freelancer is a game. Games are supposed to be fun. You don't add something because it is realistic. You add it because it's fun, and then you venture into the deepest corners of your imagination to explain why it is realistic. This mostly applies for pirates having caps. Since the Vanilla Houses used to have, like, 10 battleships each, and pirates none, now that the pirates do have a couple, we can say that the Houses have 20 or 30 or 50 or 150 each. Or 500, in case of certain Houses. That way the comparable strength is balanced.
Concerning planets, it's an engine limitation, as Thyr has already clarified. And everything will be located on the plain for as long as we don't have a three-dimensional map.