Your mistake is assuming everyone works in the field that they studied. The reality is that few people who major in psychology (to borrow your example) actually become psychologists and so on. The reverse is often true as well, people who do what you would consider more useful degrees also don't necessarily take the job you're assuming, my own example is that I did pol sci then law, and I can confirm that from my own graduating class in law less than 10% have ended up in practice (I teach, which I enjoy more personally, but I certainly have never regretted getting the law degree), similarly I know plenty of people with general arts degrees working in business and so on. The choices made by teenagers about what they want to study don't really bind them for life. The idea is that a general education at that level, in absolutely anything, has a overall beneficial impact both on the person receiving it and the workforce as a whole.