For two main reasons.
First, a game isn't only made by the developper. The publisher plays a big role and we have many examples of that. For Marvel's Spider-Man, Disney contacted Sony, not Insomniac ; the game wouldn't have been the same if it was published by Activision.
Detroit was the best Quantic Dream game and Sony is probably the best publisher for single player console games. I'm not sure the sequel will be as good without Sony.
Secondly, I have all consoles so I don't care about consoles, but I want the video game industry continue to grow. I want that PlayStation 5 sells better than PlayStation 2, I want that Xbox (2020) sells better than Xbox 360 and I want that Nintendo Switch 2 sells better than Nintendo DS. It needs many things, but I think that one of the most important one is exclusives. I want that Sony, Microsoft and Nintendo release more AAA exclusives each year, in more varied genres. And for that, they need more first party studios and more second party studios, not less.
It's maybe a good thing for Quantic Dream (I'm not sure), but I can't see how it could be a good thing for Sony.
Thank you very much.
But how many studios can make as good narrative games as Until Dawn or Detroit? Big Bad Wolf, Dontnod (they're already making an exclusive for Microsoft), RLR Training It (Sony doesn't even allow them to release their games on PlayStation) and that's probably all.