Brent (Gervais) is neither a sociopath nor the top dog he thinks he is, he's a middle manager who it's implied was legitimately good at sales, but is not at all good at the role he's been promoted into because it's a completely different one.
The actual upper management, sociopathic or not, are certainly not scouring the underlings for underperforming sociopaths phoning it in to promote (imagine Keith being promoted!), and are actually more interested in making them redundant to make efficiency savings. We don't see senior management at all, they don't see most of the employees at all and they clearly don't have much idea what's going on, initially considering promoting Brent (because he applies for it and can bluff his way through an interview) but then in the second season bringing in Neil to oversee him and get rid of him (because they've started paying attention). Neil is obviously more socially adept which is probably why he's been promoted higher at a younger age, but he also appears to be actually good at his job. On the other hand, Gareth whose career appears to have topped out at assistant to the Regional Manager, ends up getting Brent's middle management job though he has zero social skills and actually liked the guy whose seat he takes, because he wants it, he grafts and he's there. Most of the others in the office neither work particularly hard nor particularly care for seeking promotion. And it's a paper company, they don't exactly have many ways to identify high performers anyway and the really ambitious and talented people are elsewhere.
(We don't see the people at the top at all, but they probably went to the right school, started in middle management somewhere else and hopped jobs adding bullet points of performance they can claim credit for to their CV until they got C-suite titles and compensation)