We more or less just did.
The Bush administration specifically asked Congress not to declare war. I looked that up for someone several months ago who was under the assumption that Congress just wimped out. I found a report of it on Google Groups search from shortly after 9/11. There were probably a host of reasons that a declaration of war was not desired. I dont have time for more research today.
But in any case, if it's true that the reason Congress hasn't explicitly declared war is that the authorization they passed has a different legal meaning than a declaration of war, then that's proof positive that Congress isn't following the Constitution. There's simply no way around that.