Because it forces your users to make your site readable which is lazily passing off the responsibility.

Making text wider is lazy. At some point, you'll need to break it into columns so that the text is readable. Perhaps on super wide screens, three or four columns (akin to a two column book that's wide open or a newspaper) may even be more preferable.

Otherwise, the other option is to scale up thr text size, but that takes up precious vertical space on wide screens, and doesn't increase information density.

Obviously, text columns may only make sense it certain scenarios - a minimum amount of text might be required to justify a second column as opposed to just limiting the width of the first column.