Assume a $50 game. 100,000 sales. 5 gigs.
Storage and bandwidth = $28,000 (per Amazon's calculator) Cost of doing business in N countries = use paypal = 2.9%+0.30/sale = approximately $1.75/sale = $175,000
Costs = $203,000 Revenue = $5,000,000 Ratio = 4.1%
So yeah, Valve is not the shining knight here and that 30% fee is unjustifiable. Even Epic's 12% share is still very much profitable for Epic.
The 30% fee is also nowhere comparable to wholesale pricing schema of the retail model (effectively a 40-60% fee), since that involved the sale of physical copies of goods, and the retailers had to deal with issues like shrinkage and the risk of unsold inventory.
This example is also using s3 retail pricing and paypal retail pricing, which already factors in profits for Amazon and Paypal.
None of the big marketplace's costs are anywhere near 30%.
I mentioned probable economies of scale in a cousin comment, but I don't think they offer all that much. Do you have a % in mind that you intuitively think could be knocked off from the stores of Valve, GOG, Google, Apple, Microsoft, Salesforce App Exchange, and Amazon Appstore, all taking around 30% before you get into the fine print?