It's a readability thing. Many designers are taught (or organically arrive at the conclusion) that the ideal width to read an article at is roughly "book" width

It's a readability thing. Many designers are taught (or organically arrive at the conclusion) that the ideal width to read an article at is roughly "book" width. There's some merit to the idea; it's easier to keep track of where the last line ended by seeing the shape of the words without needing your eyeballs to dance all over the place. Once readability is founded, their argument is "who gives a rats arse about the rest of the page?" It's not the BEST design philosophy, but it's far from the worst one.