They mostly sell space in their digital game shop, and services directly related to that shop.

Like, when people say “split up Google” or “split up Amazon” I know what they mean: you have a bunch of things that would ideally be profitable competitive businesses, under one umbrella—Chrome, Android, ChromeOS imagine if browsers and operating systems didn’t have a market price of $0! AWS, Amazon shop, etc. Valve, I don’t see it…

There are plenty of other stores to get games from. They're just consistently worse than Steam.

Epic is giving games away but it still doesn't seem worth it to me to switch over because they lack steam input, good achievements, friend systems, good chat, inventory systems to trade items...

You need friends for a lack of friend systems to matter :)

So the thing about antitrust is that it's not the act of having a monopoly that is punishable, it's the act of using that monopoly unjustly that is punishable.

Apple's app store is a good example here--their stipulations on financial payments in apps starts to really cross the line into illegal product tying to me. Whereas what Valve has done to lock-in users to Steam is... um... you might at best point to actions they haven't taken, but fundamentally, the alternative game stores have failed because they've not really demonstrated any value proposition other than "redirect Valve's profits to us", which isn't a big motivation for consumers.

there's no lock-in in any of the contracts

Steam wins because it provides a superior product for the end-user, not because of lock-in. Games purchased through Steam can be vetted with user reviews, supported with user-created guides and steam input configurations, streamed across devices, shared with family members, and even modded; all within the Steam experience.