Why are entry-level service jobs, such as retail, fast-food, minimum-wage paying jobs so picky and choosy?

thats the definition of Capitalism

Companies exist soley to be successful and make money they are not in business to provide jobs for anyone, as you pointed out there are millions of people out of work and employers have the luxury of choosing the best people that they want to work for them, no one is guaranteed a job for any reason.

Employers want to hire the best applicant for their open position. People who are overqualified apply for lower level jobs because they can't find a higher level job. THEY have bills to pay too.

This is one of the problems with a slow job market. The concept is simple supply and demand. When hiring someone, there is really one rule that applies above all others - try to find the best candidate for the least amount of money. When unemployment is high and jobs are scarce, people with more experience and higher skills are willing to take lesser jobs for lower pay. That's just the way it goes. My only advice to you is to keep on trying. Jobs like fast food and retail are turning over very quickly. In reality, being a consistent and dependable employee is the most important thing. If you can just get your foot in the door, even for apart time job, it could lead to bigger things.