The reasons for such a curve are many.
Mostly though it is peoples unwillingness to work for themselves.
Almost everyone has a job. Not that a job is a bad thing but it severely limits how much you can make.
If you are paid by the hour, you can only work 24 hours in a day.
Most only get 8.
If you are on a salary you might get to work 12-16 hour days but your pay never reflects your work load and you don't get any overtime. This is a very stagnant income.
The vast majority of people who do make a large income either have highly skilled employees, or they own their own business.
I understand that most are scared to start their own business because most fail. But they seem to forget that they can try it again and again until they learn what it takes to make a business successful.