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.

Couple reasons:

WhatsApp was its own company, took advantage of an open market in EU where SMS (and "international" phone calls?) were extra rate charges on mobile phones. Once every one got accustomed to using whatsapp Meta took it over and now we're stuck with it.

To communicate with anyone outside of the US, where it is extremely popular and is the main interaction with many businesses as well.

It's almost impossible not to sadly, at least if you want to reach everybody in ypur contacts... It was the first popular messaging app here and inertia prevents people from moving to better alternatives now.

It's a stopgap but Teams for Linux at least gives you Wayland support, choice of seperate window sharing and background filters