probably for the same reason i don't use it much you don't actually buy anything on steam so if you bought a game on steam you only bought a temporary license and if the company that made the game ever decides they don't want steam to have the licenses to sell the game, you don't have the game anymore because you only paid for a license to play the game but for some reason they have the nerve to charge full price of what buying the actual game would have been except if you bought the actual game you wouldn't have any risk of randomly loosing it