Over time, those players would only buy one console and the older games would come at heavily discounted prices so they'd in fact be losing money whereas getting players to buy the newer games which cost like $50-60 dollars plus DLCs would mean more money for Sony. From a gamer's point of view it'd make sense to do it because you'd possibly get more users of previous consoles to transition but it isn't guaranteed that most of them would ever consider buying many of the newer titles.