Why should a doctor be paid more than a janitor?

That's the day and age we live in. Cleaning is something basically anyone can do, yet no one wants to do it because they think of it as a dirty job and no one recognises that without cleaners, the industrialised world would suffer, especially public places like hospitals and schools. Whereas for a doctor, you'd need GCSE's, A-Level's and a Degree to be a qualifyed doctor, Janitors don't require any qualifications. Especially now that more immigrants are migrating to more developed countries like the UK they will work for next to nothing just to stay in the UK whereas for most people the money wouldn't be enough for luxuries.