This is goes back to a more philosophical question. Is it fair that some get payed more than others even though they are equally skilled or maybe even contribute more to society? How do you even define being skilled?
Politicians only want to allow high skilled immigration, since low skilled immigration takes away jobs or at least constituents seem to think so.
For the purpose of a green card application you actually have to show that you tried to recruit a US person for the position, but that system is slow and easily gamed as well: You just make requirements up that only one person in the world can fulfill, the person you are trying to hire.
If you define it based on salary you exclude people who are skilled but in locations, industries or professions that don't pay well, say barbers or teachers as an extreme example. Is that fair? I don't know.
Also, the system kind of takes a simplistic view that there are X number of jobs and Y number of people who are qualified to do that job. It doesn't consider that some people are better at the job than other and that job requirements can be fluid.