A classic example of not understanding the difference between capital and income. (Even assuming your statement is correct.)
The value of southern slaves was greater than the value of all the land in the South. Southerners had been investing their profits in slaves for decades, rather than in land, machinery or buildings.
It is possible that southern landowners indeed made a greater profit per sharecropper than per slave. But over half their capital had vanished.
This had a devastating impact on southern wealth, if nothing else on its ability to provide collateral to borrow money for capital improvements. Imagine the impact on the US today if almost 50% of our invested capital disappeared tomorrow.
I can't remember precisely, but I have seen some statements from contemporary articles claiming that each slave brought in $200-300 per year for the owner, while costing him about $20 to maintain on average. Sounds profitable to me!
BTW, the South in 1860 was at the height of its prosperity. Slaves were at their highest prices ever. Secession was at root a desperate attempt to maintain this prosperity and protect $4 billion in investment capital.