Mainline vs Tube. Different rule book, traction, electrical system, signaling, etc.
There's a lot more to train driving that just knowing how to stop and start the thing. it's about taking the correct decisions in a given situation, drawing upon your rules, traction & route knowledge. The principle is the same between the two environments, but as the specifics are very different, the tube guy needs the full training course. Of the train handling aspect of it, you need that time for route learning, as much as train handling. You're also learning how to apply the rules & traction out in the real world.
The real issue for the tube driver is the drop in money whilst he trains; that's probably what he's really moaning about!
Footplate experience - look at what's actually included, rather than just the price. But don't forget that these courses have nothing whatever to do with driving - all you're doing is pulling a lever when someone tells you to, and a monkey can do that! If you want to have a go at throwing coal at something, sign on at a worthwhile heritage outfit as a cleaner. It's cheaper, you're doing something useful, and you might just find that you enjoy it.