I think there are three reasons for this:

1. Steam is not freemium. It's built on paying customers rather than advertisers.
2. PC games, unlike TV shows and console games, don't end up platform exclusives (and because games storefronts aren't a subscription, there's no reason for them to be). There are console exclusives but exclusivity has never really made sense for PC games. And without exclusivity there's genuine competition (even if none succeed at dethroning Steam)
3. Valve is a private company owned by Gabe Newell, instead of a for-profit. Shareholders are short-sighted.

That second point got me thinking about Nebula (the streaming service). I don't believe it'll be enshittification even though it has exclusives. I wonder why that is.