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).