Probably because it's enough for most people. I have a Paperwhite with (I think) 300 dpi and unless I reduce the font size to the minimum and look really close

Probably because it's enough for most people. I have a Paperwhite with (I think) 300 dpi and unless I reduce the font size to the minimum and look really close I can't see any issues at any reading distance. It feels like a printed book to me.