Aside from being a bit small and having to be held close, phones are good proportions for reading. Computers screens have gotten wider and wider, and UIs bigger and bigger, and it eats into reading space pretty heavily. Especially if you don't have a high-density screen.
In addition to more space, having only one foreground application really reduces distractions and visual clutter. Also, for some reason I am comfortable using larger fonts on phones and tablets, which makes doing lots of reading easier than on my laptop.
> Computers screens have gotten wider and wider, and UIs bigger and bigger
Sadly, most websites forcefully limit the width of the text. It's like they pretend our monitors are oriented to be tall rather than wide. Even HN has unnecessarily big margins. So unless I try to cram another window in my FHD monitor, I have ~50% or more completely wasted space. Margins should be 2-3 pixels wide, not 20-30% of the screen.