a) Because the job is much harder and more responsible than you seem to think - I was told 10 years ago that it cost £32,000 to train someone off the street to competence as a train driver, so I don't know how much it costs now.
b) In ASLEF they have a small union which is dedicated exclusively to their interests, and is very good at it - and its members support it when it occasionally calls a dispute. ASLEF _only_ represents train drivers - if they get promoted to the management grades they have to find another union (usually TSSA), and contrary to someone else's comment, very few train drivers are in the RMT, Bob Crow's union. When the railways were privatised, the drivers stuck together, and managed to "divide and rule" the privatised companies - this is how the drivers' basic wages rose from £12,000 a year in late BR times, when drivers had to work overtime to make a decent living, to a basic £33,000 - £40,000 now, depending on which company they work for - with the result that they don't have to work overtime if they don't want to (and traditionally Sunday never formed part of the standard working week and was always worked as overtime, which is why some companies have trouble running a Sunday service unless they come to a new agreement with ASLEF).
Just because you are relatively poorly paid is no justification to drag other people's pay down - instead work with your colleagues in a union to improve your conditions. To clarify, I am not a train driver nor an ASLEF member, though back at the turn of the century I was on the TSSA national executive committee - I've got a lot of time for ASLEF, and very little for the RMT.