Oh please, Sony, Nintendo, Apple, etc all take 30%+. Epic is flailing horribly while running EGS at a deficit held up solely by Fortnige skins and gambling (but it's ok when they do it right?)

Physical publishers take 60%+.

Humble bundle is getting discounted keys because it's for charity.

Steam is a publisher. They'll also finance your game with interest-free loans, you don't even need to publish your game on Steam.

More to the point, Steam offers massive value for that 30%- forums, hosting, security, patching services, hosting, matchmaking, dlc options, the list goes on and on... And more importantly, steam doesnt care if you use those services even while selling keys elsewhere. that means that steam doesn't even get 30% of the take from those keys.