Because the Army doesn't want soldiers who will suddenly fall over and need a triple bypass when they're forty? See: Schwarzenegger, Stallone, any number of roidheads with heart attacks.
Because the Army doesn't want soldiers whose muscles have developed faster (with benefit of chemical assistance) than their connective ligaments and bone structure (without benefit from that chemical assistance) - so that while the muscles look good, they're functionally risky because (analogy for the hard of thinking) you've got a 200BHP engine driving through a 150BHP gearbox? See: athletes who suffer from lots of "soft tissue injuries" when operating at full strength...
Because the Army doesn't want soldiers who (as soon as anyone's back is turned) will consistently take the easy way out rather than doing the job properly? Not vain tossers who are only concerned with how they look, not how well they can do the job?