To make sure they fit the standards for particular states, and that they don't/won't do anything harmful to others, although they still technically will be able to, by using a knife, but that's beside the point. They do major exams to make sure their going to fit in, and be a good citizen, and follow rules, and know how to speak english, read it, type it, write it, and how to do things, and what places they should go to for help, like a hospital, and car insurance.
