A dev's game being on Steam and having access to that massive customer base is much more valuable to that dev than Generic Looter Shooter #8589 is to Steam.
So some developers only sell on that other store, and then people boycott them for not being on Steam.
If you sell your game on other platforms and don’t use steam keys (e.g. distribute it DRM-free on GOG), you aren’t bound to the pricing clause. It only applies if you’re generating the keys on steam to sell elsewhere. You’re welcome to give your game away on GOG and still charge $50 on Steam, as long as you’re not giving away steam keys for your game on GOG.
Don't like the deal, don't take the deal.
I'm sure everyone would love to pay their suppliers less. Doesn't mean that they should.
Compared to nothing, yes there is 30% value is CDN hosting and a forum. Compared to a competitor there isn't - it's mainly network effects.
Or put differently, if you changed the name steam and launched them today along with a competitor that charges 15% is there enough more value to use Steam to downlad at that higher fee?
The answer is very likely not; however, that competition can't happen because of past network effects.
You want markets to compete on who has the better product, not on who got their first.
Game buyers don't care about the cut. They pay the same price. They're absolutely going to use a more polished more reliable service that actually offers refunds for defective games.
Game buyers are going to use the thing that's sitting on their desktop. Every other aspect of either store is tertiary.
EGS has been shoveling money in a furnace, solely to also be sitting on some desktops, but Steam has a decade+ head start and it's going to be an absolute slog.
