Former Head of Sony Online Entertainment on Why PlayStation 4 Doesn't Do Cross-Play
A very simple statement.

Sony has been getting slammed recently for not allowing Fortnite cross-play compatibility between the PlayStation 4 and other consoles like the Nintendo Switch and Xbox One. After enough criticisms, Sony gave a response many fans felt was lackluster and didn’t actually address the issue.
Now former Sony Online Entertainment president John Smedley, now general manager of Amazon Game Studios, explained Sony’s stance on the matter in a tweet. “When I was at Sony, the stated reason internally for this was money,” Smedley said. “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.”
Fortnite is available for PC, Mac, iOS, and Android alongside the three major consoles and the only one of them that restricts cross-play is PlayStation 4. Recently, Fortnite amassed over 2 million downloads after its initial release on the Nintendo Switch.