Calculating the value of an individual slave, each slave would be worth around $92,000 in today's dollars, PLUS the 20+ years of wages and room & board you would have had to pay a paid laborer.
Think about that. Each slave, worth nearly $100k as an asset, plus 20 years of free labor. Imagine the impact this had on the vast plantations of the South.
Imagine if you're a modern day business. You have ten slaves. On the books, you have $1 million in assets - collateral for bank loans at nearly 0% interest, as well as investment capital.
On top of that, you pay no wages. So, your competitors, who are out there paying 10 people $50k in salary plus benefits, have a nearly $1 million LIABILITY on their books.
A company with ten slaves is $2 million richer than the company without slaves literally just by existing, just on the books, and then get $500k in additional profits by not having to page any wages.
Extrapolate that across hundreds of years of pre-industrial economic growth. At a time when most other countries had either outlawed slavery, or were winding down their slave trade.