The main issue with having Open File not scroll is when there are enough Open Files that they don't fit in the sidebar. Then it isn't possible to see all of the open files, nor the folder tree. Which then leads to multiple scroll areas, and having the complexity of managing a draggable divider between the Open Files and Folders and the wasted space between the two when there are only a few open files.
In short, I think the reason this hasn't been implemented is because it is kind of messy, and has certain usability downsides, and feels less fluid than a single scroll area. I think generally Goto Files is the first-class way to jump to a file, and tabs and Open Files are just a way to visualize the open files when that is necessary.