The mediocre have just enough capability to do something right, but little enough that they're constantly surpassed by their more talented peers and never feel safe. So they go into "soft skills" that their more talented peers aren't interested in, and sandbag themselves with authority once they get it. For an extreme example, nobody is ever firing Mark Zuckerberg no matter how many obviously stupid bets he makes, and even if they did his family would have enough wealth for the next 10 generations at least.
This problem is hardly new or exclusive to tech, but we've endured enough decades of Silicon Valley navel-gazing TED talks without now trying to whitewash them as "well they're just ordinary people". Yeah, and I wouldn't trust an ordinary person to fly a plane, why should we feel good about them running companies?