This. Square Enix is my favorite gaming company (I love JRPGs, they put out games I like, easy decision
), but this is why besides the question "Is Sony even able to buy them if they wanted to?", there's no real reason to buy them out anyway. The consumer base is mostly PlayStation already and they've got timed exclusivity deals for their most anticipated titles. What they're doing now is pretty sweet on both ends, because Square has yet to put a title out exclusively for Xbox in recent memory (and they wouldn't, because Xbox is DOA in Japan). So they'll port the games to Xbox for the Western audience, but the company has a clear lean to PlayStation.
Despite people's personal preferences on here, Final Fantasy XV and Kingdom Hearts III sold extremely well. KH3's the highest selling title in the franchise to date, and FFXV's probably close to 10 Million sales overall. Final Fantasy VII Remake has sold five million copies and is the highest selling digital title in the company's history... And mind you, the game dropped right at the initial impact of COVID-19 where absolutely nobody knew what was going to happen. Final Fantasy XIV's extremely successful and is a damn near ATM machine with how well it's doing. Dragon Quest is huge in Japan, really huge. Ask Togashi what he thinks about the series.
So purchasing Square isn't a "bad deal", because they have three massively successful IPs, but there's no real gain on Square's end if they were to do it.