I think I would counter with "why are ex-husbands such jerks?"
Living off an ex-husband financially? In most states, when people marry, unless there's a prenup, partners share equally in marital assets. That means no matter who makes the money, the partners each get half.
What you call "living off" an ex-husband is actually his fulfillment of a financial responsibility to his ex-wife. She is entitled to half of everything in most states. If he's paying, it means a court has decreed he should do so.
If he didn't want to pay, he probably should have thought ahead. Prenup agreements are pretty easy to have drawn up before the marriage.