Nobody mentions it because there's no evidence for that fact being an influence on the design. There's historical evidence for the other reasons. Yours is a just-so story.
Take the oldest, crudest mouse you can find. Place your right hand on it and position it to the right of your keyboard. Put your finger on its primary (or single) button. Look at that finger from your head's natural perspective.
Your pointing/clicking finger will appear to be pointing in an upper-left direction, somewhere between 0' and 45' – exactly like the classic arrow-cursor.
Even if a designer with contemporaneous experience explicitly denied that was n influence in choosing the orientation, I wouldn't believe them. The congruence is too strong.
Oops! Indeed, I mis-perceived your comment as being a response to lotharbot's comment, about the tilt of right-hand pointing being an influence. Sorry.
However, I do think there's something more than "just so" to the darkmighty post you replied to, as well.
Anything that's hand-manipulated, and especially English writing on paper, must face the risk a dominant right-hand will partially obscure it. Thus we've got longstanding design tendencies, echoed on pixel screens, that more primary labels and information tends to be above and/or to the left, and more-ignorable (or even just defer-able) information goes to the bottom/right.
If damaging the visibility of some information for another purpose – as with an overlaid pointing indicator – a designer will thus have good reasons for allocating that damage in the down-right direction. (That is, tend towards obscuring the later/lesser information.) People immersed in our culture's visual design will even make that tradeoff subconsciously. So a "this makes intuitive sense" story, when reasoning about long-ago design decisions, is perfectly worthy of mention alongside other "historical evidence".
This is also visible in classical office planning. Nowadays people seems to have forgotten it. Look in an old office, on what side of the table is the window in most of the rooms? If the occupant is left-handed, what side is the window now? The desktop lamp? This is not a new problem, lights are supposed to be on the left side of a right-handed person when writing, otherwise you will shadow your paper while writing. Same goes with the mouse cursor, when right-handed, it's natural that it would point to the left.
