It's the tendency for people to attribute to peoples personality what is better explained by environmental factors.

Brains are neurons in a bundle, basically the same stuff as the rest of your nervous system. You learn to ride a bike for instance by repeating the task and training your nervous system to co-ordinate the whole thing. Until the neural paths are refined enough the movement is impossible because the body can't properly co-ordinate the system. Once this path is worn in it's there to stay or until it's broken and repeat which is the same process but with the added difficulty of breaking the old one first.

However due to the fact that we are experience an unexplainable phenomenon as of yet known as cognition, we experience biases in that based on the mechanics of our minds. Like how we see faces everywhere, due to our brains having a special center to process that particular patter. Which means that we tend to insert it everywhere for the pure reason it's on our minds often.

So personalities are not flexible things that can be changed on a whim like a shirt. They are the result of a social cognition loop. Where repeated back and forth echoes of that individual and society wear in a pattern of behavior we come to call a personality. This cannot be changed easily anymore than learning how to ride a backwards bicycle. Except the bike is your socialization vehicle and each persons bike is constructed from a near infinite set of responses.

So essentially we look at some people who have assembled shitty social vehicles because they only ever had shitty parts and we blame them for it. Instead of realizing the real nature of the vehicles, that they are complex constructions and require complex thought to understand.