It's business. In business deals are made. A deal wasn't made to keep games off playstation, a deal was made to bring even more high quality games to Xbox Game Pass day one, and in so doing, bolster the exclusive Xbox first party games lineup. The deal was entirely about Xbox and Bethesda with nothing to do with Playstation.
Zenimax and Bethesda chose Microsoft and Xbox. They chose Microsoft and Xbox because they trust them, because of their history, and because Microsoft was willing to showcase just how big a bet they're willing to make on enabling Bethesda and all their creative teams to do what they do better. It's simple. Same reason Insomniac chose Sony. Same reason Naughty Dog chose Sony.
And I already know what people will say, "but those studios were largely exclusively making games for Playstation for many years, so it's not the same" but I disagree. A game developer turned publisher with multiple game studios under their belt, and that makes games for multiple platforms is to me no different from an independent game studio that doesn't have as many game studios in their organization. It also isn't like Insomniac never made an exclusive game for Xbox before, they did it with Sunset Overdrive. If Naughty Dog weren't purchased by Sony they might have eventually worked with Xbox on an exclusive deal toto. I didn't bother to look to see if they ever did or not.
In the same way it can be said that Sony helped Insomniac and Naughty Dog raise their level as a game studio, and helped provide an opportunity put them both in a better space, the same can be said for Bethesda when they worked with Microsoft to bring Morrowind to Xbox at a time they weren't even sure a game like that would be received well on a console. Xbox put in the work to develop that relationship for years. The current situation is a result of Microsoft getting it right over those years.