No other app store is allowed to operate on Apple devices
That's only true outside of Europe. In the EU, they were forced to allow third-party app stores. The US government doesn't have the guts to do that, since they focus more on the needs/wants of companies, whereas the EU is really focused on consumer rights.
- Card companies take ~4%
- Patreon takes 10%
Does Patreon's cut not include payment processing?
The other thing that's ridiculous in the USA is how much credit card processing costs. Stripe is around 3%, while in countries it can be half of that (in Australia, it's commonly around 1% for debit cards and 1.5% for credit cards).
I meant more if people wouldn't have found your app without Apple's App Store. In that case, they're essentially handling marketing for you.