Tracking and driving engagement via notifications are some of the most important reasons why app users can be monetized better than anonymous web users (user sees more ads if they're spending more time, and tracking allows better targeted and thus higher paying ads)

There's another big factor with mobile app users: it's much harder to block ads in a mobile app.

Don't most users already use mobile browsers (e.g. chrome) which don't support extensions and thus ad blockers?

Firefox on iOS supports strict ad blocking. It’s not as good as some of the purpose-built extensions on desktop, but it mostly does the job.