That 30% cut is also done on the Xbox and Playstation stores. I would assume Nintendo does the same thing.
It also sounds like Valve’s price parity agreement only applies to Steam keys. So, if a developer or publisher wanted to provide the game through their own storefront or on another third-party platform then they could charge whatever they wanted.
As for the 30% cut being excessive, I don’t know if it is or not, but storing data at the scale that Valve does costs a lot of money, not to mention the costs associated with ensuring the data’s integrity and distributing the data to their users all over the world at reasonable speeds. In all likelihood they are running multiple data centers on multiple continents with 100s of petabytes of storage each with some extremely high speed networking within the individual data centers, between the data centers, and out to the wider internet. Data hosting, especially for global availability, is damn expensive.