Because it's merely a war of words, for as well armed as the cartels are: they hardly match the training and expertise of all but the newest members of the US Armed forces. Now if we actually declared war on the Zetas and other drug cartels things might get interesting. Instead of seizing drug vessels, we would just sink them. Instead of attempting to arrest cartel members, we'd be engaging them in battle. They act with impunity and have no fear of local authority, it won't be difficult to identify their leadership hierarchy and locations, and then strike. Wipe them all out in on night. Then let the small fish squabble over the pieces.