Reminds me of the monty python sketch, "what have the romans ever done for us? except sanitation and roads and canals and public health" lol.

Steam gives devs a huge marketing presence that smaller devs simply wouldn't have otherwise, it gives countless high bandwidth distribution servers that automatically scale to demand, you can integrate the largest PC social community for matchmaking or other multiplayer features, you get a community page where people can post fan content or mods, etc.

That is worth way more than 30% to most devs. The only ones who it's not worth it for are huge companies like Blizzard and Epic who can manage all that themselves, hence why they're pretty much the only ones who don't sell games on Steam.