To a bad boy, truth is made of Spandex. The only absolute he knows is that he's absolutely never wrong. Bad boys are unpredictable, unavailable, underfunded, and serially unemployed. Dating a bad boy is dangerous, exciting, and fraught with drama. The only reliable thing about a bad boy is his unreliability.
Bad boys are in demand because they make themselves scarce, thus driving up their value.
They're like half-naked people. Half-naked people are generally sexier than totally naked people because the suggestion of what could be is usually better than what actually is. Truth be told, if a woman ever got as much of a bad boy as she thought she wanted, it would be only moments before she stopped sitting by the phone waiting for him not to call. Girls who can't commit enjoy bad boys' special non-stick coating which closely matches their own. Any man, nice or overly-nice, who treats them otherwise is assumed to have something wrong with him, and is deemed unworthy of their unworthiness.