Slavery is all about bypassing the free market. The slave owner doesn’t want to pay what a free person would demand in wages to do the same work as the slave.
Slavery in the US could only have lasted as long as good agricultural land was very cheap, as it was in the early days before the country filled up. A free man, working for himself, would be able to get more out of a given acre of land than a slave who would strive to escape work when the owner wasn’t looking. This was why slave owners constantly demanded that slavery be allowed in the western territories.
The “three fifths” compromise was based on the idea that 3 free men were as productive as 5 slaves.