It's very simple, Sony wants more money, they are not making enough money with their currently exclusive strategy that they have used since the first Playstation, making games costs more and more and takes longer time, consoles have also a ceiling.
Sony sees their current exclusive strategy isn't working out for them in today's environment and are now expanding towards PC and mobile to make more money.
Sony is a company in it for making as much money as possible, they are also a publicily traded company so they have to answer to shareholders why they are not making more money.
If Sony sees that they need to put their games on PC/Steam to make more money despite hurting some part of their Playstation playerbase, they will do it.
Sony isn't a charity, it's a company whose sole goal is making money.
Also, being publicily traded (like Sony) and being private with no investors and external shareholders (like Valve) has a lot to say.
Sony has to anser to investors, shareholders, stockholders and what not every fiscal quarter, Valve doesn't. They do whatever they wanna do. Im 99% sure that projects like the Valve Index, Half-Life: Alyx and Steam Deck would not have happened if Valve was a public company like Sony.