I really think Valve have become the de-facto owners of the “don’t be evil” motto nowadays, even if they don’t advertise themselves as such.
It’s not just factually wrong to call them a monopoly, it’s uncharitable given that they are not engaging in anticompetitive practices despite being in a position (and arguably having the right) to do so.
Valve takes 30%. You can’t, in practice, sell your game on Steam and on another store at a lower price. That’s anticompetitive.
Downvote me if you want. But I recommend reading the transcripts from the Wolfire Games antitrust lawsuit against Valve before you do! They’re not a good look for Valve to say the least.
That said: enforcement on such things is not going to be 100%. Larger companies will either be purposefully ignores or make their own internal deals and contracts to follow. Some smaller games will slip through like everything else in life (Valve can still let Malware slip in once in a blue moon. I'm not surprised you can find some niche Japanese game sold for cheaper on DLsite or wherever).
But the point is that they can push thst in devs because of the monopoly. And that's how you get stuff like the Wolfire lawsuit when a few people do push back.