Most rich people get their income through work like anybody else. CEO's and the such are basically hired employees and are paid so much because people able to do the job well are in extremely high demand.
People get paid according to market principles. A job that almost anybody is able and willing to do isn't going to pay much. A job that few people can ably do and jobs that nobody wants to do, pay very well. The latter implies a certain degree of merit, talent, work ethic, personal responsibility and competence required to do it, so it pays better.
I'd buy that if you didn't get a severance package for fucking up so badly the company decides that paying you $10,000,000 to quit is cheaper than letting you run things anymore (who was that HP guy, again?). I grant that I would not do well as a CEO - I do not have the charisma that you need to be the public face of a company whenever shit happens, and I don't have the business training to actually do the job. But that latter, at least, I could potentially fix, and possibly the former given enough work.
What I do not grant is that top executive pay is determined by market principles from the company's perspective. Executives are human - they make their decisions based on what's good for
them, in the majority of cases. Each and every one of them knows that having safety nets like that as established principle is good for them, and perpetuating that is in turn what they should do. Same reason you got European monarchs that warred with France for generations fighting to undo the French Revolution. Same reason you get a Congress that refuses to improve the voting system. For that matter, the number of people who want to be and have the ability to be CEOs is, I'd wager, far higher than the number of people who are.
Now, if your definition of "can" includes the enormous slew of environmental factors in a person's life that determine whether or not CEO or Janitor is a more appropriate career aspiration, that's at least logically consistent, but it's also what we're complaining about since that
shouldn't be allowed to generate such a huge disparity.