People were not hunted down and kidnapped to be slaves
Yes they were.
They were bought and paid for
Yes, they were.
People were enslaved in Africa for a variety of reasons. East Africans were traded to other Africans and Arabs as slaves LONG before Europeans began trading in slaves for the American colonies. It was a savage culture, much like the rest of the world, only moreso. Their people were valuable to others as slaves, and the market was born.
In reponse to the OP, because there was NOTHING they could have done. Slavery was desirable in Africa because the vast majority of the people there could not defend themselves against people with guns, ships, cannon, horses, armor, money, and organization. If the Africans (who were not unified at all, but of a tribal culture) had the ability to wage war on a European scale, the Europeans would have gone elsewhere to obtain labor.