This is only mostly true at Playstation, not Sony as a whole. They have purchased more than a few film, television, anime, and music publishers. Also, when they first entered the market, they did pick up Psygnosis, who was both a developer and a publisher. Recently, they've concentrated more on picking up independent studios, usually ones that they had already published games and own IP from. But they aren't new to buying publishers in the abstract. They simply haven't done it recently IRT video games. Unless you count Bungie. They self published Destiny 2 after their contract with Activision ended, and all of the expansions afterwards. They also used to publish other companies games before they became a Microsoft studio, but stopped once that happened.
Oh, and Sony also own stakes in several publishers. Epic, From Software, Scopley, Devolver... Sony invested in all of those, and maybe more, in the last few years. Sony also owns things that own things. Like the Game Show Network, owned by Sony Pictures. GSN has a games division that publishes games. That's a pretty deep rabbit hole, though. I don't think I have time today to research all the stuff they own that own a video game publisher.