Because sometimes I do wish to use the same browser window.
Case in point: Oracle documentation insists on using a targeted link at certain places. I don't wish to have a billion windows around. Or if I do allow the target, and it's a *named* target, I end up with the most recent click appearing over whatever I had navigated in that named window.
Since I don't use IE (nyah nyah nyah) -- or Windows for that matter -- I just middle-click on the link when I want it to open in another window. No need even for a context menu. :-)
While I do appreciate the time you spend trying to make it a better experience here, my own personal preference is for no targets.
Of course, the ideal solution here is to add a user preference and parse the targets out of the links when necessary so everyone can have their favorite behavior... Bribes are acceptable, liquid bread preferred. ;-)