Any individual can get a better job. But not every individual. There aren't enough good jobs, and society would collapse if all the retail workers, bin men, teachers, nurses, etc suddenly became software engineers.
Okay, but you're disregarding both new hires and seniority. I know a garbage man who's a poet and makes $100k a year, with health care and a pension. Other people who have that job for a year or two might decide to move into a different field. There are more people being born all the time. They start somewhere and move on. I served when I was in my 20s. Now I'm served by people in their 20s. So?