Because that's not beneficial for companies. They want to make (more) money.
The only option most developers and publishers would have is to move to another store, where the cut is usually the same, with the exception of Epic Games Store. And as pointed out elsewhere, setting up and managing your own store ends up being more expensive than a 30% cut. And then you still don't have the same features as Steam.
Unlike publicly traded companies, Valve is not beholden to shareholders, so they, unlike most others, are in a unique position to not JUST maximize profits. I think it's okay to point at Valve as an example for other companies to be more like, because most are still worse. But obviously we can always strive for better, as well.
(Also, out of curiosity: Under a capitalist system, can you have anything BUT a capitalist company?)