Because a declaration of war as defined by international treaty (which Constitutionally we are required to honor if we agreed to it) ties our hands. It limits us to specific states and specific actors. An 'Authorization to Use Force' against violations of the laws of nations is Constitutionally sound (Article 1, Section 8, Subsection 10) but doesn't tie our hands in fighting a specific country, but instead, going against a movement that is not tied to a specific nationality and doesn't fight under a single flag.
The President does not have a private army at his disposal. A declaration of war unites the country. We should declare war on every country we bomb before we bomb them. Not like a Pearl Harbor. Bush already said any country that harbors terrorists is a terrorist country. So a WOT is just a war on words. involve Congress, Declare War, Unite the Country.