... why? I imagine most of them are doing the same activity as a bus full of people reading the newspaper or a magazine, just with new forms of media...

As an older person, I have to say that it feels palpably different to me.

Old time buses with people reading felt a little more like some 80s buses full of people with Walkman headphones. The material was a bit of a performance of "I'm minding my own business".

A modern environment with people grafted to their phone apps feels more than that. It sometimes feels more like the movie cliche of an opium den.

Here, people aren't just signalling the idea that they are minding their own business. They really are lost in their own world of addiction. A circus clown could ride down the aisle juggling flaming bowling pins and these phone users would not even know.