Just what is an internship? When I was in school everybody got Summer time jobs. Those usually would pay for o e semester with Dad and Mom paying the rest. But that was in a time where one job would support a family.
Internships are a form of on-the-job training. It lets people get some experience on their resume before they leave school, so that they (presumably) have an edge once they are out in the job market. Being able to apply to a position and say that you already know the basics of the role can be the difference between landing the position and having to keep looking.
Some places, it's a form of slavery.