The OP said only employers know, and this turned into University math majors being highly-paid, and so forth.
From the suggestion that only employers know the precise details of the job market (insider speculation), we can conjecture (fairly accurately) that increasing the production of math majors is likely to produce oversupply unless the employers are responsible for controlling the increase *and* have a stake in that increase (that is: if they have to pay for it, and so are inclined not to produce too many in excess). A segue into the virtues of obtaining a math degree without that context suggests people should independently seek math degrees, which starts to lean toward oversupply.
Remember: people read into what you say, and ignore what you omit. When you omit details like that, you make more complex arguments than just the words you put down. In the context of North America and most of Europe, people believe increasing college attendance and decoupling that attendance from employment (and from the responsibility of employers) gives people freedom and wealth, and so will read into it as such. That's why you get ridiculous arguments for funding more STEM degrees so more people can get good jobs when our current STEM degree holders can't find jobs.
The political farce surrounding the workforce development system built around college is a sore spot for me. In truth, it's just that people blindly repeat dogmatic axioms like "access to college promotes equality and gives minorities upwards mobility" without actually understanding how the economics work. They look at developed countries--countries which have been growing in wealth and creating a demand for a skilled workforce--and see more of the workforce going into skilled employment, and conclude that this is happening because of college, and not that the economic growth has lead to a larger workforce development institution and broader employment in skilled labor. They ignore all confounding variables and just make the simple conclusion. In this case, the simple conclusion is actually so backwards as to be harmful to society, putting people into effective serfdom and creating more poverty. I have a cringe reaction to people being so bluntly wrong about things, and this one gets stepped on *all* *the* *time*, so I'm sensitive.