The 30% cut was industry standard for digital distribution for years. Google, Apple, and numerous other players all took 30% as standard.

That being said, Steam hasn't taken a flat 30% for years now - their standard agreement starts at 30%, decreases to 25% after the first $10m in sales, then decreases further to 20% after $50m.

Furthermore, Valve has done more in terms of providing services, APIs/libraries, and end-user features (all with no additional fee to the developers or consumers) than any other game storefront has. I'd say they more than justify their cut.