Why didn't the American ancestors simply hire African workers instead if enslaving them?

they weren't American ancestors, they were immigrants
blacks sold blacks as slaves

Free labor was one reason.  Investment value was another since people reproduce and the children of slaves became the property of the slave owner.  
The whole idea behind it is disgusting to almost all people today and even some when it was happening.
Agricultural workers today don't exactly make much money either. Your idea is idealistic but very naive

Slavery was universal throughout the world and had existed since the beginning of civilization.  It was simply an extension of the way it was.  In Europe, slavery ended in the middle ages and was replaced with "serfs".  The laws were such that the serfs were the property of the landowners.  Russia was the last country to end slavery in Europe.  It was the communist revolution.  Most people do not realize that.  Unfortunately, what followed fell far short of what could have been.  Likewise, most people who criticize the past and the way it was are typically people who have never done hard, physical work for very little reward.  It doesn't take much of that to begin to think it would be better if the country next door to you would do the work, especially since you don't like them anyway.