That's the thing though- a lot of them aren't even smart enough to care to divide X by Y, as you say. Not that they can't grasp the concept, they just don't give a shit. Zero foresight, and an inability to look even months into the future, let alone years. A lot of it is a lack of education and/or good parenting, and a lot of it is, as GusFrerotte mentioned, the Hip Hop Culture so to speak. Every single hip hop video/song you see/hear nowadays talks about nothing except jewelry, cars, money, drinking at the club, and other shortsighted possessions that do nothing to build wealth, rather just bleed it. But hey, if that's cool, trendy, and will get you laid and recognized, why not right?
Also, a lot of these guys who end up broke genuinely thought they were doing the right thing by investing and diversifying, only to be taken advantage of by attorneys, agents, representatives, etc who fed them BS promises of high returns only to milk them for every cent. Gotta surround yourself with the right people.
When I think about what I would do with wealth like this, it really, really annoys me to hear about these wasted fortunes.