How? Because parents, teachers and mass media said to 'do what you love and the money will follow' or some variant, so kids picked majors with no demand in the labor market.
Because affluent, well-connected kids got humanities degrees at Ivy league schools and then used their connections and the prestige of the school to secure high paying jobs, and kids from blue-collar backgrounds didn't realize they can't do the same thing with a humanities degree from the local college.
Because kids who should have gone to trade schools or entered the workforce after high school were convinced they had to go to some college, so they wasted a few years studying something they're not good at and don't care about, so they didn't learn anything to make themselves more employable than they were before they started.
Because affluent, well-connected kids got humanities degrees at Ivy league schools and then used their connections and the prestige of the school to secure high paying jobs, and kids from blue-collar backgrounds didn't realize they can't do the same thing with a humanities degree from the local college.
Because kids who should have gone to trade schools or entered the workforce after high school were convinced they had to go to some college, so they wasted a few years studying something they're not good at and don't care about, so they didn't learn anything to make themselves more employable than they were before they started.