Greed mainly. Oh and queue jumping are to blame in my opinion. Its also relatively easy to become a pilot. Its a process of self selection afterall only limited by your ability to find the cash to fund the training.
In medicine I can't wake up and decide I am going to be a doctor and head off to a lecture the next day. There are barriers to entry to ensure I have the requisite level of skill and acumen needed to hopefully pass the course. In aviation anybody can wake up and decide they want to be a pilot. They can take as many attempts to pass tests as the rules allow and then fire off as many CV's. Hell they might even strike it lucky and pass the sim ride and interviews. Then a bit more cash to fund a type rating and hey presto.
Problem is that we live in a culture of "I want it now". TV and society tell young people that they can have everything they ever dreamed of without waiting. No need to live with mum and dad and save up for that first house when you can rack up extortionate amounts of debt and declare yourself bankrupt and come back for another shot in a few years time.
Taking those sentiments to aviation its easy to see that along comes someone like FR and says give me some cash and I will give you a shiny big 70t jet to fly. Don't worry though cos I am going to pay you sh*te but in just two years you will have 1800 hours and you can toddle off to a proper airline that pays a fair wage and has T&C's and get all your money back ten fold.
Problem is there aren't any of these "proper airlines" anymore and the industry wide T&C's will never ever get back to where they were.