Why? Assuming no-one else has, why should Sony be any different? Cross-platform play is a part of modern gaming, very much so now that games run on mobile devices as well as consoles. Sony either are the one player who doesn't join in, or they follow the way culture is going, and in doing so ensure they don't alienate consumers. The way to win this war is to be the one platform everyone wants to be on, and not to be the one platform existing users are afraid to leave.
My next console could either be a console that also lets me play my games on tablet with tablet owning friends and family, or a console locked out of that mobile space. That significant enough to be a buying decision. So better to keep your customers and then have them buy through your portal than lose them entirely.