That has nothing to do with phones and everything to do with readability. It turns out, people have trouble reading overly long lines of text, so website developers tend to limit text to a certain width. It’s also a little bit of carryover to pre-responsive design when websites had to work well on 800x600 desktop screens, as well as 1080p screens, but that hasn’t really been a thing for many years now.

I like the second answer here:

I agree with the user Jared Farrish: it’s to make the content more readable. If a paragraph spans the entire width of the browser window, it can be taxing on the eye to move from the end of one line to the start of the next line if the paragraph takes up many pixels in width. Many websites tend to limit the width of the page for this reason. In addition, some Web sites use media queries to change the font size if the user’s browser window width is very large.