Mouse and keyboard should be alternatives on equal footing as much as possible. The gutter for page up/down has the benefit that you don’t have to move the mouse all the way up and down to the arrow buttons when repeatedly switching between the same adjacent pages, or when you positioned the scroll button slightly off where you wanted the position to be, and adjust it a little by paging up or down — especially when the scroll range is so long that a small mouse move on the scroll bar corresponds to more than one page.

For scroll ranges that aren’t overly long, you can long-press so that the area ends up scrolling to that position. Only for long scroll ranges is it necessary to drag, use the context menu, or the Shift key, for absolute positioning.

There are certainly trade-offs, but it’s not like there aren’t justified reasons for the way it is.