Netflix stopped offering subscription through the app (last year IIRC) for this reason, though they grandfathered existing iOS-mediated subscriptions for the obvious customer retention reasons.

Also I don’t think you can subscribe to amazon prime via the iOS app — you certainly can’t buy video via the prime video app!

You CAN buy video using the Amazon Prime Video app, it is in the category where Apple gets a 15% cut of the sale.

The funny thing is that switching to non-Apple payment is hard (I get that that's Apple) and the support staff of Netflix was really helpful in making me switch.

It's actually really annoying, implementation-wise. Apple subscriptions are read-only for third parties, which creates some annoying hurdles where you have to warp your data model to accommodate the fact that someone could have two subscriptions to the same thing at the same time, which normally you wouldn't want to allow them to do.

My guess is that Netflix is grandfathered in under long contract. It’s also a predictable source of income year after year, which is far more valuable than an unpredictable app by a smaller developer. It also definitely enhances the value of iOS more than most apps out there.

Ultimately, large corporations can get side deals because they wield more power and influence than smaller companies.