True, but you have to start somewhere. It's possible that in the example, one could become a teacher, then quickly rise up the administration ladder. Or, they could start in teaching, and write a successful book on education, or develop an educational software tool. Those are all possibilities of having one's salary increase, but you have to have a starting point.
It's just like buying and mortgaging a house loan. The bank might not know what you may earn 10 years from now--it may be much, much more that you earn today. But they will offer you a loan at today's salary.
Depends on the career. Nurses start high but salaries do not grow much at all.
Lawyers often start low and grow tremendously.