I think that's actually (partially) a more pedantically correct rendering. The scrollbar isn't depressable, although the buttons on either end are. Clicking the "current location" widget portion of the scrollbar doesn't do anything, so flat isn't wrong there - it's clicking in the non-inhabited zone that jumps ~a page up or down but that's not really captured by either the Windows 95 or the Windows 2000 aesthetic, if memory serves. Windows 95 got the depressable buttons at the top and bottom of the page right, though.

I think "just for fun" is probably a large part of it, but a stated reason is also that it makes tighter vertical integrations and consistency across all layers easier.