I don’t really understand how it could be economically viable to keep people fully enslaved with effectively no rights. It seems like a tremendous amount of work would have to go into keeping those people subdued, if you’re using slavery on a large scale. This could apply to any instance of slavery but I’m mostly thinking about the context of slavery in the Southern US. I don’t see how they were able to easily keep all those people enslaved, especially considering how many slaves there were. I know that some areas had more total slaves than free citizens.
From what I have read, slavery in the Americas was largely adopted due to the desire to grow sugar cane. Sugar was a coveted luxury item in Europe and could make you rich very quickly, but apparently it's extremely labour intensive and the tropics had dangerous animals/diseases, so finding enough paid workers to work on your sugar cane fields was hard or next to impossible. Later in North America, the coveted crop was cotton, which is also labour intensive and really not attractive for free labourers to come and work for you.
So they turned to slavery. Having to pay only for your slaves' food and shelter instead of the high wages that would entice a free labourer to work this shitty job, was apparently economically viable. Especially so since most owners don't seem to have bothered much to feed their slaves well or provide good shelter and clothes.
Fields slaves would often be going in rags, building their own houses on their half day off on Sundays and tending to a vegetable garden to supplement their meagre rations. When there was no field work, owners would also lend out their slaves to other people and get paid for their work.
Escaping from slavery was also not as easy as you might think, especially for those slaves that were far away from areas without slavery. There were patrols on the streets just stopping black people and asking for papers from their owners that allowed them to travel.
Since slaves couldn't read and had little access to information, they might hardly even know where they were and certainly have no map.
And even if you were young and wily and thought you might just walk at night hiding from pursuers, your owners (and paid mercenaries) would literally hunt you down with horses, dogs and rifles. Many people also had family such as old relatives or small children that couldn't possibly make such an escape trip, or who would have to be left behind.
Not having any money and/or food, was also a factor. How would you feed yourself/your child once you were free?
If you are interested to read about how some slaves managed to escape (and what kept others in slavery), I can recommend the book "The Underground Railroad" by William Still, written in 1872. It's free on archive.org and Project Gutenberg (and not to be confused with the recent novel by Colson Whitehead). The author's parents and one of his brothers escaped from slavery and he was born free, later helping to shelter escaped slaves and writing down their stories at the Philadelphia Anti-Slavery office.