I think that:
Not many people can actually perform this job.
This kind of job requires really solid skillset across various domains.
It feels like you not only need to be great at managing people, doing business, doing politics (the real one), having vision, but also do not be bullshitter, so your employees believe in you.
Meanwhile, CEOs strategies aren't something you can test within a month or two and fire CEO and get a new one.
In some industries you need at least a few years in order to see whether specific "bet" was good, so you aren't going to fire your CEO after half of the year.
Meanwhile CEOs decision can have very very costly impact on the company, so the risk is huge.