I don't know if Sony explicitly pushed single-player games more than that was what their devs happen to want to make single-player games. In the past they seemed to push on the online aspect of games. I think they even said at one point they wanted all their games to have an online component. Which was what I think lead to the user-generated content in Infamous 2, Uncharted multiplayer, God of War multiplayer, and Ratchet and Clank multiplayer. It was at a time that many publishers believed that multiplayer was how you get more new game sales which was further enforced by online passes that Sony took a part of. There was a rumor around that time that the reason Socom was having so many problem was that Sony kept pushing them to try to be more like COD. Future/maybe-present, the last head said they want to do more multiplayer. So maybe that's why Horizon is rumored to have co-op.