For all of the 'America' doesn't necessarily mean 'USA' talk-- Forbes has been producing the Forbes 400 'richest Americans' since 1982. If it has been 'wrong' for over 30 years, you'd think at least a couple of these people would have have brought it up to Forbes for correction, rather than the comments section on a tech website...
If Bill Gates took his $66B and split it up between the ~10M US households that are at or below the poverty line, he'd essentially be handing each one of them a check for $550 a month for a year. That'd be really nice and all, but does it really fix the problems that face the low income families? I'm guessing their bigger issues are high unemployment + lack of available jobs + lack of appropriate skills for high paying jobs. I'm not sure that $6600 in a year and then nothing beyond fixes that.
Even ignoring that most of the money isn't really available to hand out; it is tied into assets [houses, jets, cars], so you first need to find someone willing to pay the $20M value of Bill Gates' house [or whatever it's worth], to have that cash to hand out in the first place. A lot of it is probably in stocks and a major sell-off would be bad as well.