Why are text articles on the web so thin?

I've recently noticed that articles on the web tend to be very thin with lots of whitespace to the left and right. Why is that?

Two examples:
http://99percentinvisible.org/article/underwater-cloud-inside-cables-carrying-99-international-data-traffic/
https://www.theatlantic.com/technology/archive/2017/04/the-tragedy-of-google-books/523320/