Because it's Apple's phone, Apple's operating system, Apple's app store, not the developers. Does Disney allow lewd content on Disney+ or at Disneyland?
No, it's my phone, that's why I had to pay hundreds of dollars for it. If every video I watched on my computer had to be approved by Disney I'd be just as outraged.
Then go ahead and install YOUR operating system with YOUR rules and YOUR apps.
Yes it’s your physical device and it’s exactly like they sold it to you with a license to use Apple’s software (iOS) on it and everything that comes with it. It was YOUR choice to buy an iPhone or something else.