Cross-platform play is a much more common feature for modern games than it was in years gone by. Whether you're talking Xbox and PC, PC and Switch, PS4 and PC... there are a number of games where people on all sorts of platforms can shoot, stab and strategize against one another, though notably not between Xbox and PS4. That's also the case with the latest title to receive lots of cross-platform attention: Epic Games' Fortnite, and according to Microsoft, it's all Sony's fault.
“Microsoft has long been a leading voice in encouraging the adoption of cross-platform play and the potential of connecting players across PC, mobile and all consoles,” Microsoft told Kotaku. “We’ve worked closely with Nintendo to allow cross-network play between Xbox One and Switch and our offer to do the same with PlayStation players still holds. For any other questions regarding ‘Fortnite’ cross-network play between Xbox and PlayStation, please reach out to Epic or Sony directly.”
Purportedly, Epic and Sony did not reply to requests for comment. This would be par for the course though. The same issue happened with Rocket League in 2016, which saw a huge influx of early players thanks to cross-platform play between the PS4 and PC. When the Xbox One was added as a platform though, it was not linked up with the PS4 user base, despite it numbering in the hundreds of thousands.
As Kotaku points out, CD Projekt Red is only waiting on Microsoft and Sony to do the deal and it can setup cross-platform play between Gwent players on Xbox and PS4 too.
Could it be that because the PS4 has massively outsold the Xbox One, that Sony doesn't want to aid Microsoft game player counts? Back in the Xbox 360 era, Microsoft, which had the more successful system for a time, wasn't as interested in cross-platform play then. Draw your own conclusions on that one.