When Apple first released App Tracking Transparency, I immediately used it to block the trackers

There seems to be a common misconception that this blocks trackers, which is not the case. Use a DNS-based ad/tracker blocker and watch the logs and you'll see that many apps happily track you. As far as I understand, ATT blocks is cross-app/website tracking. If you deny, the app does not get access to the Identifier for Advertisers, meaning that tracking services cannot use a single identifier that is used across apps. While this initially had a large financial impact (see the article), trackers have probably developed other ways to correlate data from apps/websites now.

The real solution would be for Apple/Google to offer an option to completely disable in-app trackers and if an app would violate it, boot them from the App Store.

Of course, they would never do that because they make a lot of money from targeted advertising with their own ad networks, so either they would have to block themselves or get in hot water with regulators.

Put differently, Apple and Google are not your friend here.