The number I’ve seen floating around a few places is that app users are, on average seven times more profitable than web users. Reasons include:
The app being on the device acts as a reminder to the user to interact more
It’s easier for an app to send notifications to get users to open it and interact more (Android has reduced this by requiring permission; browsers required it long before)
There are more limited options for blocking ads in an app
There are more opportunities to collect data in an app
Are there any good reasons for it, too? Security, maybe?
Security for the user? Probably not. “Security” for the developer in that they can prevent people from using the app in ways that aren’t profitable? Likely.