Consider the situation we are in now, thanks to generations of Leftist programming in American public schools.
A large percentage of our 18-year-old's simply would not qualify for the draft. They are either catatonically ignorant, obese, diseased (often "in the head"), substance abusers, or in horrible physical condition. Furthermore, they have not learned any reason why they should be prepared to fight for their country, and would piss and moan so pathetically that the screeners would simply allow them to drift off into their mothers' arms.
Civilized societies in this age should have compulsory public service, whether in military form or otherwise. But since we are not civilized...
Never mind.