Other idiots also made the scrollbar thinner, harder to see, and hidden unless you purposefully move your mouse cursor to the side of the page. With a mouse and good eyesight, it's not that big of a deal, but I would imagine it's an accessibility issue looking for a place to happen.

What OS?

Even without the arrow buttons I can scroll without dragging, just click the scrollbar wherever you want.

They did it because the buttons took up room, were used by nearly nobody, and (in the case of the "jump to top" / "jump to bottom" buttons) destroyed the user's scroll state in a non-undoable way that was too easy to trigger.