The primary reason is when you train intensely enough to stimulate growth, you're also saturating your system with CORTISOL.
Cortisol is an overpowering catabolic protein that literally 'eats' lean muscle tissue while preventing bodyfat metabolism -- you might say it ranks right up there with binge drinking, smoking cigarettes, and eating sugar in terms of the damage it wreaks to your efforts to get lean and muscular:
1) Recent studies have shown a direct correlation between high cortisol levels and abdominal bodyfat distribution. It literally prevents the release of fatty acids in your abdominal region, increasing lipid storage in this area.
2) Just as disturbing, research shows cortisol literally prevents rapid muscle growth from happening after you train due to its 'catabolic' (muscle-eating) effects . It makes the gains from intense workouts few and far between. In fact, every time you encounter stress of any kind, your body's "fight or flight" responses kick in, pumping out cortisol -- smothering muscle growth and bodyfat oxidation. It's like throwing a wet blanket on a raging campfire.