Lack of competition. No other app store is allowed to operate on Apple devices. They have a monopoly, which is the only reason they can charge as much as they do
Sure, Apple are providing a payments platform, but why do they deserve 10x what Stripe charges?
I don't believe that 30% cut includes payment processing. That's an additional fee
Sad to think about how little creators actually get at the end of the day. Example of a $100 donation/subscription:
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Apple takes 30%
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Card companies take ~4%
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Patreon takes 10%
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Taxes take another big chunk
After all that a the creator would have, what, $40?