Answer:
Because there is not enough.
Step-by-step explanation:
People always say our world is cruel and cold and that is true. Our World and society needs to support billions of people, if it was compassionate to every single person we would soon find ourselves running out of supplies. This also applies to jobs as well. Lets say there was two jobs, Job "A" and Job "B". Job "A" pays a lot, has good insurance and a good retirement plan. But Job "B" gives mediocre pay and has no insurance and no retirement package. Obviously people would flock to Job "A" and nobody would go to Job "B". This is a major problem because soon there would too many people with Job "A" and no one would go to Job "B". The only way to fix this is to restrict Job "A" and only let the best of the best to join. All the other people who failed to get Job "A" will be forced to get Job "B". This is a balance that is necessary because everyone can't just work one Job, in order for this world to function we need different occupations to work together and co-exist. Like for example lets say architects were Job A and construction workers were Job B, if when a person at adulthood is given the choice to become either a architect or a construction worker then most probably everyone would choose architect and this would be a problem because we would have the architects to plan the building but no construction workers to build the actual building. The only way to fix this is to bottleneck the architect occupation and make people make a choice, "Do I want to go to college, study hard and procure debt to become a architect" or "Do I want to get a Job right after High School and be in no debt and become a construction worker".