It's easy to take a gesture as intuitive as pull-to-refresh for granted. That simple downward pull to reload content really only came into being once smartphones started becoming a thing: it was only used for the first time by a Twitter app called Tweetie back in 2008. Before that, we were mostly reduced to tapping a little 'reload' button to refresh content. Yuck. Twitter eventually acquired the app and filed for a patent on the gesture, though it promised to only use the patent defensively. Anyway, that's a little history on the now-ubiquitous gesture, which I'm sure will come in handy at your next cocktail party.
The thing about gestures, though, is that they really only work well on a touch interface. That's why, on a desktop, you probably click on a button or use a keyboard shortcut to reload content instead of trying to perform some convoluted gesture using the mouse or trackpad. Of course, Chrome OS inhabits that strange space between traditional desktops and touch-enabled devices, so while it has some of the quirks we're used to on PCs, it also supports touch interfaces — and so it makes sense to optimize its interface to work with gestures.
As spotted by redditor lucasban, Chrome OS now finally supports the pull-to-refresh gesture, though it's only available on the Dev channel for now. However, the gesture isn't turned on by default, so you'll have to enable the relevant flag (you can copy and paste chrome://flags/#pull-to-refresh into the URL bar to do so) in Chrome's settings menu to activate it.
Now that Android apps work on Chromebooks, we will likely continue to see Chrome OS become more and more touch-friendly.