Because there's nothing common between those names when you want to speak of them collectively. Right now, "Linux" is it, if you want that changed, I'd suggest going with the existing movement to call it "GNU/Linux".
Not good except to inflate Stallman's ego. GNU software makes up a small part of the average Linux distribution (which includes the kernel and massive amounts of other non-GNU free software). It'd be like saying a Volkswagen should be a "Bosch/Volkswagen" because some of the parts are made by Bosch. Using your logic, we should call it Apache/BIND/Nmap/sendmail/.../Linux. My Photoshop disk came with some nifty third-party filters and utilities, so should I prepend those to the Photoshop name?
One main question to ask is: Can Linux run without the GNU tools? The answer is Yes.