Retailers demanded a piece to put games in their stores as well. There has always been a price paid for going to market using someone else for it.

They allow devs to sell on other platforms and provide them steam keys for free, bypassing that commission.

I see this point come up all the time when it comes to Steam, but I have yet to see anyone really propose an alternative. How much should it cost to host your game on Steam? It obviously can't be free because of hosting costs, and you're also paying for marketing and discoverability, so what's a good price for it?

Until recently, 30% was the industry standard for large software stores. Google is apparently lowering its cut after losing their recent battle with Epic, so it's possible that the industry standard changes. I'd hope that Valve adjusts with it.

You lost 60% when selling physical media in stores, 30% was an amazing deal when steal launched