For the same reason that rich and handsome guys don't marry ugly and poor girls.<br />
Let me put it in another way. There are two reasons. The wealthy partner has their own way of managing their financial assets. It's unlikely that the poor partner will have the skill and experience to manage the couple's assets to the degree that will satisfy the wealthy partner.<br />
As for looks, people generally marry only attractive partners. Why would anyone marry someone they don't find attractive enough for sex? Okay, for their money, yeah, but that's usually the incentive for the poor partner.
Would you marry a girl with a face like a digger bucket if you were an international playboy ?
to cheat on her while you use her crippling self esteem issues to make her do all the dishes and **** you want no part of.
Same reason that a dog licks his b@lls.
Thats a narrow view, many men in third world nations that would be considered 'ugly' by western standards are married to super desirable women, its all a matter of taste. Western women are mostly disloyal parasites which trade their well used flesh for goods and services, so they cant be the standard held to all women.