I think people are not as rational as they seem and it's basically a form of power hunger - PHBs feel more powerful when the distance between them and their employees is larger: one feels much more powerful having 10 people well below him than having 1 person a little below him, and 10x programmers that also have social skills in a company where IT matters will have a say in lots of things, even not IT related and even if not in a direct way, so they end up closer to the PHB on the power scale than
each of the 10 1x ones, which is a bad feeling... Now if the 10x is totally lacking in social skills and introvert, that's ok for the PHB, but if lacking in social skills and more of an extrovert, it's bad again, because although he wouldn't have a say in things because he's "the smart asshole", he'll make everyone,
including the PHB, feel and
look stupid (and they probably will be 20 iq point below him and they'll know it and feel somewhat demoralized by him).
Basically the 10x guy is uncomfortable for all guys above him (not only in software but in any other field that has a 10x skill gradient) that lack self-control, self-knowledge and people skills (what's called "being wise"), and most PHBs are "very not wise".