Nothing of what Steam provides costs 30%. Discoverability and free marketing only provided to games that are already successful and have hundreds of wishlists. That's only possible to achieve if you game already have it's own following and community.

12+ years ago if you released on Steam it was a big deal and platform provided traffic to everyone, but today it's flooded with games so basically you're on your own.

The only thing that allow Valve to charge this much is network effect. They are not vendor-locked platform like App Store, but they do have nearly monopoly on PC.

Little correction: obviously not hundreds of wishlists, but hundreds of thousands. You need at least 100,000+ to even be considered to have a successful Steam page.