Clinton - it's like religion. Some people have a need to believe in this sort of thing; it's a touchstone against the apparent randomness and inchoate nature of existence. No amount of logic is going to dislodge that belief, any more than an atheist can convince the devoutly religious, because belief lies beyond the realm of logic.
And even if you were to shake someone's belief in astrology, they'd probably replace it with something else.