The screen isn't too small to use web browser presenting site that looks just like the app, obviously. It is too small for comfortably reading or using the browser in general, so using the phone's browser for searching stuff on google is something that many older people simply don't do (in my anecdotal experience, I have no hard data to back this claim up). Searching for the train times in city z, going into a web site and finding that it is actually identical to the app and just as useful is something that is very unlikely to happen to my mother-in-law.

My 80 year old dad struggles trying to type on his phone. He uses voice to search YouTube videos for sermons, music, how to videos and to make calls.

There are a lot of websites that don't scale well with small screens.

Just the simple trick of zooming in on a website with a "2 finger pinch/pull" is something a big part of the population doesn't even know how to do. I know my mom would probably just give up.

Buttons on many websites are way too small for some people to use.

Those are never an issue on native apps. The UI on apps scale with the size of a phone's screen. No matter if you have the smallest iPhone or the largest Android.

This alone is enough for many people to not use the web browser unless absolutely necessary.