Probably a more accurate analog to the cartridge would be the storage and distribution services. That takes some infrastructure in networking and storage.
It's not really the same because the reason Valve built out all of this infrastructure was so they could take the middleman's cut. Basically, GameStop buys games cheaper wholesale and resells them at a higher retail price. Valve gets all of that by being the direct retailer to the end consumer.
Not even having to manufacture a physical good is a separate cherry on top of that.
Apple actually gives developers 1PB of free storage per app with CloudKit, and it even has some nice privacy features for users. Of course it has limitations, so most developers prefer rolling their own backend.
Consider a game that holds a leaderboard on steam, or any app that stores user progress on its servers.
Not exactly pertinent to your comment but pertinent to the conversation as a whole:
If Apple/Google/Steam stored all app data for developers and also did the marketing for developers, that is closer to what 30% is worth imho. For hosting the platform, yes your grandfather was the first to stab a flag into the digital real-estate, and for that I owe you 30%? Feudalism that manages to shift cost to both consumer and developer at the same time -- impressive, but not very noble.
That's not a good analogue for a cartridge, though. The cartridge or the device itself stored data locally. Modern gaming consoles store their data on HDD's. Anything beyond that is entirely developer's prerogative.
And according to another user, they do offer storage through CloudKit... which it appears they do, included in the dev account/App Store distribution model—1 PB: https://developer.apple.com/icloud/cloudkit/
