THey dont declare war , because they would
1. have to have set conditions for victory
2. would have to win in a timely manor ( draftees tend to get pissed off if it drags out too long , families of draftees start to get unruly at home over it )
3. have something to show for the money and lives spent .
4. They would be too afraid of the population if we lost ( we have never lost a declared war , we have not won a war sense 1945 )

As it stands now , its an all volunteer military , thats why the outrage of the past decades by the public isnt there . The percentage of the population effected is very small , its a planned event by the individual , nothing is disrupted . On the other hand , the draft effects ALL of the population , people are pulled away from employers , pulled away from families , etc. against their will . Now , you will see outrage from the population if things dont go well .