It's because the vast, vast, VAST majority of people have no idea that many apps are just showing a website. Also, the app version is almost always more efficient in terms of precious phone screen real estate compared to a browser. Apps also remember who you are so you don't have to login. It isn't hard to understand why people like them.

That said, many apps are horrible from a privacy perspective. But that is largely hidden from the average user, most of whom simply don't think much about online privacy anyway.

I hope the ubiquity of irritating ads are the thin edge of the wedge that gets more people interested in ad-blocking, and then perhaps online privacy more generally.