They do have competition, and apparently publishers are willing to pay that fee. Also, it's more like 20-25% for larger games (IIRC 25% for sales >$10M, 20% for sales >$50M).
I think GOG is still 30%, and they seem to be losing money even with that cut, and EGS apparently still isn't profitable, so I really don't think 12% is sustainable. Valve might be able to do it, but that's because they have massive market share.
Why is it so high in the first place? What’s being offered in return?
- marketing
- Steam platform features for users (e.g. Steam Input), Steamworks for developers (e.g. DRM, multiplayer, achievements, etc), and things like SteamVR
- platform support (e.g. Proton for Linux, Linux driver development, etc) - devs don't need to do anything to support Steam Deck
- regional pricing - so publishers don't need to think about it
If publishers felt they were being ripped off, they could go elsewhere. We've actually seen some big names go off and make their own platform to keep more of the revenue, but then they came back. It turns out Steam offers a fantastic service for users, publishers, and developers.
Other platforms like EGS and GOG don't offer anything close to what Steam offers, which is probably why Steam still retains a massive marketshare without doing anything anti-competitive like paying for exclusives or bribing users w/ free games. They literally just offer a premium service and charge market rates for it.