Ignorance of economic systems.
Because most people don't view the job market as an exchange of goods and services. You're quite literally selling your labour to someone that's willing to pay for it. Therefore just like any other goods/service there is value attached to it based on what makes it unique when compared to other's on the market. And also the demand for good/service.
I largely blame primary education for this. They regularly spread the myth that all college degrees are created equal and going to college will get you more money. Which isn't true, specialization is what will get you more money.