Because that does not always seem to work..... some quests either have prereqs (that you would presume the other boxes would have) or it just fails with a "can't share quest".

Second reason is I have enough macros as it is - keeping others to a minimum or delegated to the mouse is my goal.

What you propose is a workable option though but it would not work for other things like right clicking on a quest giver or targeting somebody in combat. Hence the need for a mouse per machine