Momentum scrolling has a capped velocity†. Your velocity with momentum scrolling approximates a sigmoid. And the cap is far too low to actually get somewhere, if where you want to be is “somewhere in the last few chapters of a book” and you’re currently at the beginning.

Scroll bars, on the other hand are perfect for that kind of jump — where you don’t know what you’re looking for, but you know mostly where it is, and you’ll recognize it when you see it.

† There was a time, right when momentum scrolling was first introduced as a third-party mouse-driver feature ca. Windows 98, where it wasn’t capped. But that was a horrible UX: if something was really far down the document, then by the time you saw the thing you wanted to stop at, you would often be going far too fast and overshoot it by a lot, requiring almost as much scrolling back in the other direction. This first iteration of the feature basically expected people to understand how to “land” on a target using inertial, frictionless thrust. I imagine there were some future Kerbal Space Program players who enjoyed that, but most people didn’t.

Hmm, I'm not sure. On macOS at least if it is capped it's capped at something very high because I don't think I've ever had an issue getting through something long.