For exactly the same reason slavery exists today. For personal gain.
Africans traded gold, ivory, and other resources for goods such as firearms, textiles, and rum in the triangle trade. The main commodity that Africans were traded for in this trade route was slaves.
People were captured by Africans and enslaved. They were then taken by their African captors and sold at the coast for Manillas (copper and bronze armlets) that had been manufactured in Europe. The African kingdoms that ran the slave trade became very rich on the proceeds of selling Africans that they captured.
The kind of works that the Africans do for Europeans are farming crops. The Europeans donâ??t usually trust the Africans for them to be the middle man that is why they are usually placed on farms.
The short and simple answer is that they were not seen as humans, but as savages. The Africans that the pioneers found in their travels were usually simple hunters. They were capable of building only simple structures, hunting and gathering, and growing crops. Their social structures, if any, were unsophisticated compared to Europe and Asia. The pioneers saw this as a great opportunity for very cheap and abundant labor. They proceeded to ship the Africans back to America and made them do all of their hard labor.
The kind of works that the Africans do for Europeans are farming crops. The Europeans donâ??t usually trust the Africans for them to be the middle man that is why they are usually placed on farms.