I remember reading a study saying that middle management had the most stressful jobs. The worst stress is caused when you have responsibility without power. CEO's have a lot of responsibility, but they also have a lot of power.
For starters, unless you've got a lot of executive experience, if you had to turn up to work tomorrow and be the CEO, you'd be awful at it. So would I, so would anybody else that doesn't have that experience. I guess this wouldn't evenly apply to early stage start-ups, but they tend to not have much executive leadership anyway, regardless of whether they have a "CEO".
Being the CEO is all responsibility. There's very little room for error, and you unavoidably have to deal with a myriad of issues that you will have no personal interest in whatsoever. It is absolutely the most high pressure and difficult job in an organisation. If it weren't, then anybody could do it, when obviously that's not true. Also, if you're measuring who is the most stressed in an organisation, then you're going to come up against a lot of selection bias. You'd expect CEOs to have a better responsibility to stress ratio, because you'd expect CEOs to be better at managing stress.