Perhaps one reason people buy iphones, is because you can't install from outside sources. It's part of the feeling of trust in iPhone. If you get a subscription in-app, you can easily cancel it on your phone, without having to worry about going through some kind of retention hell (call to cancel, dark ux patterns to prevent cancellation, etc...) As well as level of security in that it's one less company with my credit card to worry about being leaked/hacked/whatever.