Don't forget they're net figures and the US may be high in both directions (strong pull and push factors, not least because the "millionaire" category in wealthier countries includes a lot of mid career professionals at multinationals)

France is also an obvious destination for wealthy Francophone Africans, and has freedom of movement with 27 countries (whereas I think it's quite easy to have a net worth of a million dollars and still not get a US visa)