Why are scrollbars always on the right?

Most people are right-handed so it's more comfortable (for most people) to have things activated by the right hand to be on the right-hand-side of the work area.

Having said that, many apps DO have the facility to locate the scrollbars on the left margin instead of the right margin. If they don't, it means the writer of the app did not do a good job of her UI design. And some apps even use a left scrollbar as the default.

Amongst the xterm command-line options we find:

       -leftbar
               Force scrollbar to the left side of VT100 screen.  This is the default, unless you have set the
               rightScrollBar resource.

       -rightbar
               Force scrollbar to the right side of VT100 screen.

I always specify -rightbar when launching xterms from scripts.

My theory is that it is because of the margin, which is pretty much always a straight line. We are more accustomed to there being wider space at the end of a line (except in justified text, which is more common in print than on screens where a wall of text can look pretty obnoxious.)

Geography aside, the scroll is on the same side that you thumb to change pages in a book.

Putting a scroll bar on the right doesn't make us feel like the margin is in the wrong place, or that you're changing "pages" from the wrong side of the screen or window.

Having said that, are Arabic scroll bars on the left? I've worked in SA, but I never noticed since I used my own laptop.

I imagine the sideways scrollbar would be really counterintuitive on the top vs the bottom.

Considering vertical toolbars with buttons can be on the left or right, I'm not sure it's a focus or handedness issue. I would guess someone picked it and there was never a good enough reason to change it later.

What is apparent though is that in the latest UI design trends people aren't even clicking on vertical scroll-bars enough to justify them being visible by default, and I presume they're mouse-scrolling instead, so perhaps left or right never really mattered much at all?

Are they? https://en.wikipedia.org/wiki/Scrollbar#History_and_progress...:

“Between 1981 and 1982, the Xerox Star moved the scrollbar to the right to get it out of the way and reduce visual clutter.”

Edit: original question said “on the left”. That page has examples of systems with scrollbars on the left, though (I don’t think I’ve ever seen scrollbars on the top of a window)

You mean the right? I think it's a consequence of people being mostly right handed. It's just easier to reach things to the right over reaching them on the left, for example when sifting through a stack of papers. Similarly on touch screen devices, holding a phone on your right hand means it's easier to reach for a scroll wheel located on the right side of the page opposed to the left one.

I suspect that it might be because we use a left-to-right writing system, so our eyes start on the left. You want their eyes to start at the content, not at the scrollbar. So you put the scrollbar not in the first place they look.

This may be the same reason that the horizontal scrollbar is on the bottom, not on the top.

In the western world, text runs left-to-right. So when people are scanning a webpage, after they have read the content, it is natural to move your eyes or hand to the right to get to the next piece text. This is analogous to reading a book, where you grab the right side of the page to turn the page.

all the (western) world is oriented for right-handed people. I think this is the most important reason for this. They will work in the same manner on left side, but its too late to change this behaviour.

you mean on the right?