Because it must include labor in other countries where the pay scale is very different. A barista at Starbucks in NYC is going to have a different income than one in Vietnam. By making these all US companies, you are comparing the CEO pay of an American against an average of global norms which are far lower.

It would be useful (to me) to include another column that's just the median pay of US-based employees.

Another interesting tact would be to show what a Vietnamese CEO makes versus their company's median pay, or what this looked like in the US in 1950, for example.