In actual fact, only about 5% of all African shipped to the New World came to what is not the United States. 95% went to the Caribbean or Latin America.
And slaves in those areas were mistreated much more brutally, as can be easily seen by the differential reproduction rate. Slaves in the Caribbean and Brazil were worked to death so quickly they had to be continually replenished by new imports just to keep up their numbers.
Meanwhile, the slaves in North America underwent a massive population explosion, equivalent in size to that of their white masters. AFAIK, this is the only slave society in history to even reproduce itself, much less grow in size.
The US slave owners generally wanted the slaves to breed because it was cheaper than importing them I suppose. The same way you wouldn’t kill off the cows, horses and field hands - just bad for business.