Yeah, but do they do this on purpose (e.g. to avoid promoting someone who might outshine them), or they just do the random thing because they are incompetent?

Yeah, but do they do this on purpose (e.g. to avoid promoting someone who might outshine them), or they just do the random thing because they are incompetent? I suspect there is a lot of bias in action, for example extraverts genuinely believing that other extraverts are competent even if they are not.