Sony have a massive lead which is partially self sustaining on the basis that, as far as multiplayer games go, people buy a new console to play with their friends.
Why would Sony want to give this advantage away?
It would be ideal, but it's not really fair to expect that from a company in a highly competitive market.
No-one knows how well the X1X will do and it would be unwise to give that console another advantage so close to its release. Sony need to be able to say "the PS4Pro isn't the most powerful console, but it's still where your friends are."
Maybe it will happen one day, but it's hard to settle on the best timing of such a move. Maybe a year after the X1X has been on the market? They'd know its sales momentum and would only be a year away from launching the PS5 to take back the performance crown. But then the XboxTwo will only be a year away and will have the advantage of greater performance whilst its only direct competitor doesn't have a real advantage in terms of a greater install base.
I'm coming to the conclusion that, as long as Sony and Microsoft are locked in competition in the same space, merging the online communities can only ever be detrimental to the dominant company.
Maybe a means of making it more enticing to all parties would be to insist that an individual must pay for Xbox Live or PSN if they want to play with members of those communities? Even then though, the sales momentum and market dominance issue still remains prohibitive.