There is in fact an enormous industry of software and operating system features designed to make it as difficult as possible for the user to circumvent.
But not as difficult as possible for the user to be aware of. If an antivirus installed itself on my computer without my knowledge and hid the fact that it existed from me, we would call that malware.
Apple doesn't have a provision that makes it difficult for the customer to use other purchase methods, it has a provision that makes it difficult to inform the customer that alternate purchase methods exist.
And again, there is no reason for Apple to care so much about that unless they think it's beneficial to Apple for the customer not to know what choice they're making. Apple's policy is not the equivalent of an antivirus program or a security measure, Apple's policy is the equivalent of a car salesman trying to make it impossible for you to comparison shop or look up competing prices.
Because "you'd have a worse experience" if you used a competitor. Sure. That's definitely why Patreon can't inform users in the app that they're paying 30% more per-subscription. /s Also I'd have a worse experience with another Internet provider and that's why Comcast needs to make it difficult to compare price rates with other companies. Come on.