Apple provides an almost-always guaranteed refund process for purchases made through the App Store, usually no questions asked.

No way do I want to trust randoms with my payment info.

Hell I just purchased a Claude.ai Pro Subscription and there's no way to remove my card info afterwards. No way to contact support (the useless chatbot send button is grayed out).

If I recall correctly the major proponents of the push for external payment systems on the App Store were companies like Match.com who own Tinder etc. and indulge in various scummy user-hostile practices (like charging certain demographics higher for the same service). Sure, break the "walled garden" and let the wolves in.