Because I'm clearly not talking about starting salaries here, that's why. How many teachers do you know on 20k?
There are lots of jobs that are well paid and have short training periods and don't require academic qualifications. Train drivers elsewhere in the country train for a year and start on less than 20k. Air Traffic Controllers, 6 months training then they quickly work up to 50k (they also have effective unions, note), oil rig workers (the non-engineers, obviously), that's just off the top of my head. Police, for another.
Everyone thinks their job is harder than a train drivers. I think I would very quickly be able to learn to drive a bus. They don't go that fast, you don't need to plan when you're going to brake miles in advance, you can stop quickly.
Dealing with the public doesn't really phase me (been there, done that). I can count change from fares. If you break down another bus comes along and takes your passengers and other cars and buses can drive round you, you don't block the entire line. You don't need to get out and fix your bus in a hot dark and dangerous tunnel while under pressure because people are trapped until you do. If you knock someone over people will be about to help and you'll get help quicker than the train driver, on his own in the tunnel. Train drivers have to learn routes too, and they change. And they are accountable when they are late and get in trouble. And they also are a mistake away from losing their job, a massive amount of pressure. If a bus driver overshoots his stop by a few metres it's not the end of the world. Train Driver does that? Disciplinary.
See? It's easy to pull apart a job as being a pile of piss when you've never done it.