The problem is that it's the only option across the OS. There's no reason this option couldn't exist with other options, built by the app developer so it cost Apple nothing.

The problem is that the fee is not charged to the Apple user on a line item in addition to the content price. Like all markets are supposed to operate. It's information hiding.

Do you know the markup at any retailer? Maybe in aggregate if I look at the financial statements or some specialty retailers like car dealers?

I can find that price by bidding down or calculating it. Retail is no mystery.

I thought it was 15% for the first million in revenue, then 30%.

But that's not designed in a way which considers that despite the app bringing in millions in total revenue, each individual creator makes far less than that.