Slavery in the US colonies evolved from indentured servitude (both European and African). Once the debt was paid, the indentured servant was freed but generally did not become prosperous as the best land was already taken. The participation of former indentured servants in uprisings like Bacon's Rebellion led to the switch from indentured servitude to racial slavery. I'm surprised Megas hasn't brought this up, since the presence of the Natives led to the institution of slavery in the US. Slavery didn't become a legal reality in the US colonies until around 1650, about 150 years behind the rest of the Americas.

Politically, the plantations were not going to be broken up when the US was formed since many of the FFs were plantation owners themselves.

Look to Zimbabwe to see what happens when you split up a large working farm and give it to people that don't have the proper knowledge to run it. There is more to farming than dropping seeds in the ground and harvesting the results.