I believe this is why companies look for Junior Developers who actually know enough to be a mid or senior developer.

One day, a company that doesn't have the technical chops to know the difference between python and ruby hire a developer who is still in school. That developer doesn't know what he's worth, so the company gets him for pretty cheap. He does amazing things, takes last minute requests, learns some along the way, but eventually leaves because he just got contacted by a recruiter telling him how much he's really worth. He leaves, but the company needs to fill his spot. The company asks the former rockstar all the technologies he used to accomplish his job and throw that into a job description. The company could only really afford the junior so they keep all the stuff about being a junior, but because they need to maintain all the hodge podge stuff the previous developer put in, they need someone with experience enough to jump in.