If you only have a few widgets and a few widget areas, this doesn't matter.
But for those of us who have dozens of widgets, and a theme with dozens of widget areas, we need the widget list (on the left) and the widget areas (on the right) to be scrollable independently of each other.
The way it is now, if you have a widget on the bottom of the list and want to put it into an area at the top of that list, it is a big hassle to drag it there.
Put each segment into its own limited-height div, with each div having its own vertical scroll bar, and presto! Problem solved.
We would then be able to view the widget we want AND the widget area we want on the same screen, and have an easy drag.