Why is it not possible to get attribution for web-to-app campaigns if the user does not opt in via ATT?

In order to get device-level ad data in an app, you need to get consent via the AppTrackingTransparency framework. For app-to-app campaigns, if there’s no consent, Apple has provided SKAdNetwork as the alternative attribution method.

However, for web-to-app campaigns, there is no alternative that is equivalent to SKAN. This means if the user does not consent in-app, then there’s currently no way of attributing that ad (either in aggregate, at a device-level).