For people wondering how this is possible:

> In the Caribbean, Dutch Guiana, and Brazil, the enslaved death rate was so high and the birth rate so low that they could not sustain their population without importations from Africa. Rates of natural decrease ran as high as 5 percent a year. While the death rate of the US enslaved population was about the same as that of Jamaican enslaved persons, the birth rate was more than 80 percent higher in the United States.

> In the United States enslaved persons were more generations removed from Africa than those in the Caribbean. In the nineteenth century, the majority of enslaved in the British Caribbean and Brazil were born in Africa. In contrast, by 1850, most US enslaved persons were third-, fourth-, or fifth-generation Americans.

> Slavery in the US was distinctive in the near balance of the sexes and the ability of the enslaved population to increase its numbers by natural reproduction. Unlike any other enslaved society, the US had a high and sustained natural increase in the enslaved population for a more than a century and a half.

The US prohibited importing slaves in 1807-08 so a forced breeding program evolved to continue slavery in the South.

https://en.wikipedia.org/wiki/Slave_breeding_in_the_United_S...

The last point may be the most important. The US banned the importation of slaves in 1808. The enslaved were treated like livestock and that's why they still had slaves in the 1860s.