Ask HN: Why readability is ignored?
There are lots of researches on what is the ideal line length is but sites like wikipedia or HN still let the lines be too long.
Many sites (including both Wikipedia and HN) practice responsive design, where the width & layout of screen elements adapts to the size of your window. There's a good argument that providing the user control of their own situation outweighs the consensus research on the "average user". After all, you're trying to make each user happy, you aren't trying to make a hypothetical average user happy.
Most of the time, users skim sites, they don't read them. Optimizing for reading is counterproductive when the user just wants to find a sentence or two and go away.
Sites can become aesthetically unpleasant if they have a small column of text and a gigantic amount of whitespace. Nobody reads the site when the user bounces.
Sometimes sites are set up to use indentation to convey hierarchy, eg. in comment threads. These can also look awkward if the right margin keeps shifting over with each new comment. (Although personally, I prefer that to having tiny little comments that are 2-3 words wide.)
There's a meta-point here: oftentimes, academic research into one specific area is not relevant to a larger product, because there are other trade-offs that have to be made. When I first went into software, I wondered why all this cool academic research into programming language design was being totally ignored. It turns out that oftentimes the really cool language features like macros, type inference, predicate dispatch, tail-call optimization, etc. have side-effects that make them impractical for use in a real software system.
Unlike printed text that is finished finished, text on the web changes per device, time of day, etc.
You might be very interested in https://www.readability.com/ which allows a person to instantly morph the majority of articles to their set reading standards with a plugin or bookmark.