Because that would be vague and misleading. If I'm asked "What's the current Debian?" and had to answer in 2 words, I would most likely answer "Debian 4.0". But even if users would understand that current is in relation to upstream software, calling testing "current" would be misleading ("Why the hell does Debian call glibc 2.3 "current"?")."testing" isn't what desktop users would want to use - at least not by that name. They don't want to test things. They want current packages that aren't too broken or have too obvious bugs. Why not call it "current"?