I agree with the fact that 30% is a lot, but the way you've written it suggests no one but Valve takes a 30% cut. 30% is pretty standard for game platforms. Steam, GOG, Apple's App Store, Google's Play Store, PlayStation's PSN, Microsoft's Xbox Store, and Nintendo's eShop all take 30%. The only reason why Epic Games takes less of a cut is because it's the only way they might entice games to get published on their platform.

Steam is super problematic in so many ways, I just don't think their 30% cut is as big a deal as everyone thinks it is.