30% for this is high, but then there is also the discoverability. Which I think does beat google by long way. So they probably would not have sold as many copies without popular platform.
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.
This is probably true on balance, but needs to be tempered with the reality that they also pioneered or popularized many of the worst parts of modern gaming. Always-on DRM, paid DLC, loot boxes and exploitive monetization, esports gambling (indirectly, they were complicit until legal pressure forced them not to be), FOMO monetization, "early access" and launching incomplete games, etc. All exist in their modern forms at least in part due to Valve.
Disclaimer: I'm a valve fanboy who buys all their first party software and hardware. They still put out great products despite the ways they've changed gaming for the worse.