Maybe I'm a small ignorable minority but almost all my library skimming is done on a phone. I explore new technologies when rocking babies to bed or dozing off myself.

And yet, desktop browsers have no more need of hijacking the browser's scrolling behaviour than any other browser.

And leave scroll bar rendering to the browser or OS vendors!?

Think of the pixels!

/s

Seriously though I believe it's often done to make infinite scrolling and scroll-transitions more visually pleasant. (Not that I agree with the sentiment, but if visuals are more important than anything else...)