For the same reason almost all relevant ui-elements are left-aligned across operation systems and apps: because we are tuned to process things left-to-right and the mouse is more often on the left side than it is on the right, and it feels less intuitive to move it to the right end of the screen.
- window-operation buttons on macos (close, maximize ...)
- application-menus in gnome, macos etc (file, edit, view ...)
- heavy-used buttons in browser (page back, refresh ...)
Except for the scroll bar. I vaguely remember from many years ago (maybe it was on Windows or some Linux desktop at the time) that you could put it there. NextStep seems to have used it like that by default, judging from screenshots.
- window-operation buttons on macos (close, maximize ...)
- application-menus in gnome, macos etc (file, edit, view ...)
- heavy-used buttons in browser (page back, refresh ...)