This would only make sense if there were only two platforms.
In reality, you have multiple platforms, and all those platforms are supporting cross-play among them, but one of them isn't (at least not with all of them). That makes the outlier's platform less appealing. The developers want to do cross-play, and the other platforms are doing it among themselves besides Sony, people really think Sony is going to come out on top of this vs the developers and other platforms? It is less appealing to buy a cross-platform multiplayer game on Playstation if all the other platforms support cross-play for the game(s) but the Playstation doesn't.
Also it's so blown out of proportion of how advantageous cross-play is to other platforms, the games are already multiplatform, what needs to sell a multiplatform game is the quality of the platform of choice for the consumer, restricting cross-play is worsening that, not retaining some advantage.