Slavery built very little. It is/was a failed economic system, which was already in relative decline against the free-labor practices of the industrial north.
It was a typical forced, crony-capitalist system, which benefited a few wealthy people and governments, but besides the slaves, also suppressed wages for poor whites.
That slavery “built America” another ignorant, emotive leftist, race-mongering slogan which is exactly WRONG.
Which is why they get upset if you say, "We should have picked our own damned cotton."