There's nothing special about phones (in this sense). Most people just aren't interested in interacting with the other people around them. Or that might just be the famous British Reserve in action.
The book ends.
You may be engrossed for a while, or a long while, or even a very long while, but eventually you get bored. You have to close it for a while and look around, or talk to someone, or just sit there.
A phone with internet is infinite. You get bored of (a) and then you can go to (b) then (c) then back to (a) for hours and hours and hours and hours on end.
Very good point, thanks for this. Might indeed be first and foremost simply a personality trait (they would have been "introverts" and reluctant to conversation also in the pre-smartphones era, or even in the 19th century).