In 1794, the US banned using domestic ships to import slaves, followed by banning importing slaves entirely in 1804. So without being able to buy slaves from overseas, the traders turned to breeding and selling slaves born in the West. By 1860, there were still just under 4 million slaves in the US, one quarter the amount ever sold from the Atlantic.

There's also that much of the warfare and enslavement in the African nations was fueled by Europeans. It wasn't something naturally occurring on the big scale that Westerns just took advantage of, but something turned into a big practice for profit, hence one contemporary statement: "All the old writers... concur in stating not only that wars are entered into for the sole purpose of making slaves, but that they are fomented by Europeans, with a view to that object." (Slave Trade Debates 1806, Colonial History Series, Dawsons of Pall Mall, London 1968, pp. 203-204.)