Because they made the game? If you think that buying a game means you "own" it you've never really paid much attention to your games. For just about every game you've ever installed on your computer, before you're allowed to install, you are required to acknowledge a ream of text as having been read and agreed to. That ream of text is your list of rights as a buyer of the game, of which there are relatively few, and that agreement is binding. Just because you ignore it does not mean it never happened, it just means you're dishonest. Game companies could very well prohibit secondhand sale if they had the means and the will, of which they have little. That does not mean you're within your rights to participate in it. If game companies were interested in selling you the license to their game, they'd be charging you much, much more.