Trains don't drive themselves as such (except on completely segregated systems - DLR and the LU Victoria line being two prime examples). The Driver is still driving and, especially on DOO services (plus DCO with no Guard present) is singularly responsible for several hundred passengers, especially if there is an emergency.
One thing I've learned from driving on a heritage railway (Class 37 locos definitely don't drive themselves!): it's [relatively] easy to make the train move. Getting it to stop safely and smoothly in the exact spot on the platform every time is a lot harder!
So there is traction knowledge (how the traction works and the ability to fix faults that develop when in service while the line controller is screaming at you to get moving because the penalty payments are racking up quickly), route knowledge (speed limits, junctions, signals, controlling signal boxes), rule book knowledge (defines everything with respect to how trains are worked including what to do in an emergency like protecting the line etc) plus the job itself (in many cases lone working, and shift working at unsociable hours, then the trauma having "one under" i.e. seeing someone go splat when they commit suicide). Yes a lot earn £40k+ but it's a bit more than just pushing a button and letting the train drive itself.
That said, the driver in question - if he was distracted by something like watching pr()n on his phone/tablet while in the cab (never mind actively driving) - is in for some serious disciplinary action. Not paying attention to your driving is inexcusable.