Web attribution assumes a web checkout
Classic affiliate tools attribute by dropping a cookie when a link is clicked and firing a pixel on the merchant's checkout page. That works when the purchase happens in a browser you control. It does not work when the purchase happens in the App Store, because there is no page on which to fire a pixel and no cookie that crosses from the browser into Apple's purchase flow.
Fingerprinting is not an option
Some mobile attribution historically leaned on device fingerprinting to guess which click led to which install. Since iOS 14.5, using fingerprinting for attribution is against Apple's App Tracking Transparency rules. That closes the workaround that once papered over the gap.
The fix is Apple's own signals
The reliable path is to use signals Apple itself provides: a StoreKit 2 appAccountToken carried into the purchase, and App Store Server Notifications v2 that confirm it server side. These are deterministic and privacy-respecting, and they see exactly the conversions that web tools miss.
This is the core reason a commission-only affiliate platform built for the App Store can attribute installs and subscriptions that web-first affiliate tools structurally cannot.