Why does this have to change now? Why does the platform need a cut?
The point is that you could distribute your software any way you wanted to without permission or license fees paid to the maker of the computer or its operating system.
If there's a new store on the iPhone, virtually no one will use it.
The default is good enough, it's already installed on ~1Bn devices, and most of those devices use it somewhat regularly already.
Not saying 10 or 30 is the correct amount. It's just different now.
Android could also be paid as a part of the purchase price of a smartphone and maybe it already is, but of course a one-time payment would be much less profitable for Google than a continuous stream of revenue from the apps.
To meet shareholder expectations of continuous growth.
UPD: At least while ago in 2019 Epic claimed to give a choice of payment processor to use for games on their store: https://www.gamesindustry.biz/articles/2019-12-06-epic-games...
Running a store means deciding what goes on the shelves. The only features that Google could use to decide whether they want to sell an app are the features that are apparent after installation. Your argument would suggest that they shouldn't be allowed to curate their content at all.
Other comments describe a few specific examples of that abuse (like also thwarting Epic's attempts to be preloaded by OEMs), but I think it's more interesting to look at how money is changing hands as evidence of market power being abused.
In particular, consider that Google in theory should be fine accepting payments as low as their hosting costs (and probably much less since their app store helps attract phone purchasers to fuel their ad network, since they also take a cut from OEMs, etc). Compare that to each individual developer on the store who in theory is able to afford at most some large fraction of revenue for the privilege of being on the store. Note that the latter number is (usually) much greater than the former and that the latter is what's actually being paid. If there were any real competition for something as trivial to build as an app store then you wouldn't expect Google to be able to successfully price discriminate across so many orders of magnitude of prices and extract nearly all of the surplus value from the end user purchasing an app. The fact that they're able to do so and are doing so should serve by itself as a strong indication that something suspiciously anti-competitive is happening. My $20 app wasn't 20x harder for Google to host than your $1 app, so why are my fees 20x higher? Because they can get away with charging that much because they stomp on any competition.
That isn't any kind of nail-in-the-coffin proof mind you, but it serves as a useful heuristic for spotting abuses of market power in many markets. Throwing that same idea at the right to repair movement, the amount paid to John Deere for tractor repairs is many times greater than what people have been able to achieve on their own (when they've been able to do so), and the only barrier is DRM that was added to prevent unauthorized repairs. If John Deere were really that much better at repairing their tractors than third parties then they wouldn't have to artificially exclude everyone else from attempting it to retain market share and command those prices.
Nobody's saying that Google or John Deere shouldn't be allowed to profit from the things they make, but using existing power to hamper competition and seek greater rents isn't good for society as a whole.
When people claim that the app store fees are excessively large they're usually comparing to payment processor fees, but neither major app store functions solely as a payment processor. So if not that, what are we comparing to when we say the fee is high?
