Most websites exist for one purpose, and one purpose only: show text and comments to that text. Often that is reflected in their mottos and mission: A List Apart's "focus on best practices", Medium's "welcome to where words matter", Reddit's "conversation starts here", and the list just goes on and on and on.
It only feels natural for these sites to go to extreme lengths to hide as much content as possible, cut off text, replace text with "calls to actions", floats, animations, jumping around. Right? Right?!
In the screenshots above:
First row: A List Apart (there are several pages of text in that article), Twitter, two screenshots from Medium
Second row: Reddit, Reddit after you click "view more comments" on the first page, IE 8 with various toolbars enabled.
You'd think only one of those is a joke. I don't know anymore.