Finding qualified people to drive these trucks are actually quite hard. There's a lot of misconception that you can apply for these jobs and start making $150k+ immediately (and somewhat the media helped fuel that perception). The applicants need to show both qualifications, and more importantly, experience. And the vetting process is very rigorous.
I have a friend who drove mining trucks in a Copper Mine in British Columbia when he was a student. He told me it was the most mind blowing boring job except for the time he almost fell asleep and drove it over a cliff. From his description, it didn't sound like a particularly high skilled job - but perhaps the type of Mine he was in had a different type of truck system.
'Vehicle and driving' are actually one of the major risks in mine operations, for the reason you just described. Fatigue, overconfidence, complacency, or simply momentary lapse in focus can lead to dire consequences.
There are controls put in to ensure safety (e.g. cabin sensors to detect signs of driver fatigue, in-vehicle monitoring, vehicle blackbox, etc.), but at the end of the day, it can only help so much.
Disclaimer: I work at Rio Tinto.