It's because X-Window, the original Unix (and thus Linux) desktop system, supported 3 button mice WAY before Windoze did. It used it for the clipboard paste operation; you highlight some text in one window, and it's immediately put on the clipboard; then when you middle-click, it's pasted into whichever window is under the mouse pointer. Most old hand Linux and Unix users like this behaviour.

It's been optional, and configurable for a long time. It's mainly controlled by the receiving window's configuration, but you can set it globally to do just about anything supported by your version of X-Window, including to scrolling. It's been like this since about the late 1990's, but it's just not the default behaviour, probably because for much of that time, most Linux users preferred the X-Window behaviour.