Because it's Apple's marketplace and Apple's customers.

They spent billions on hardware, software, R&D, marketing and operations.

Why would they give another business a free access to a billion of their customers?

You want to earn money of Apple's customers? Then pay Apple a revenue share of the money you get from their customers.

That's how pretty much any marketplace works in the world.

From your local Target to Salesforce/Shopify/PlayStation/Epic Store/Steam/etc.