how does an 18-year-old who is in college and has either a small part-time job or no job at all, get a credit card, but an 18-year-old with a full-time job and not in college get rejected?
Two ways, I think. (I got my first credit card in just about these same circumstances...)
First, the person in college is assumed to have good future earning potential. The company figures that before too long they'll have a good job and will be able to pay their bills.
Second, the person in college probably has parents who will be helping the student. I think that the companies assume that a kid in college is going to be able to get help from their parents if they need to.
So, in the short term they have Mom and Dad to help them and in the long term they'll probably get a good job. So the college kid is a good risk for the company while the 18 year-old with the job might not be.