Plantation owners in america often argues that enslaved africans were better off becuase they did not have the mental capacity to tun their own lives. today this seems both racists and ioiotid. how could such racist beleifs flourish?

It is undoubtedly wrong and a very racist mentally. As it offends all the humanity as one. Each human being holds the same capacity as any other being inside a society. But, the plantation owners were not the only one to blame for having such an ill mentality but it was mainly due to the factors which made them to think in such a way. The people around them when they were little treated others on basis of color they had, so they were wrong and this is just false that a person of a specific race having a white skin has an edge over the others. As they were brainwashed by their society and elders to consider black people as inferior to them in regards to mentality level.

Explanation:

As we know that there are different factors which makes up a person character. And some of the factors are which makes up a person is due to the up bringing of an individual in a given environment by his or her elders.

Along with that the factors from the social interactions with other beings has a great influence on a person character as a whole, so it was all some unfortunate acts which lead them to think in such a racist way about others.