Because work ethic is not taught in high school or university (or in American families/society). And many 3rd (or higher) generation Americans have an unhealthy sense of entitlement.

Many American teens are told that they are able to become and do whatever they want and that money, knowledge, education and job stability have very little utility for happiness.

At age 18, "whatever they want" means partying while gaining large amounts of debt and barely passing through a ridiculously easy, but useless liberal arts major.