Why do random people show up in my dreams?

They are figment of your imagination, but some of them are people you'll likely meet in the future

Usually dreams use symbols and such instead of a direct message, in your dreams. So, it means that when people show up in your dreams it doesn't usually mean that your mind is referring to those people in particular but it means that your mind might be trying to refer to people in reality who have something in common with the random people in your dreams.