In that it all comes down to money, and the company position is that it doesn't want customers to feel like they can play games on other platforms. John Smedley, the former President of Sony Online Entertainment, took to Twitter recently to state just that. Which, has since been removed.

But not before it was widely circulated.

btw when I was at Sony, the stated reason internally for this was money. They didn't like someone buying something on an Xbox and it being used on a Playstation. simple as that. dumb reason, but there it is.
— John Smedley (@j_smedley) June 18, 2018

Cross-play is something that gamers have been dreaming about for quite sometime now - where an artificial barrier put up by hardware makers has been the reason why we don't have open platforms. Rocket League supports cross-play, but that's actively blocked by Sony when it comes to PlayStation 4. Minecraft is in a similar situation. The reason why this issue is making headlines post-E3 though comes down to the additional measure Sony placed on Epic Games' mega-hit Fortnite whereby not only would cross-play not be supported but any account that would attempt to log into the game on another system would be blocked.