Step 1: Find them sitting in the right hand lane, not overtaking, with nothing next to them in the left hand lane.
Step 2: Get right up close behind them, encouraging them to move over, to no effect.
Step 3: Give up, pull into the left-hand lane, and overtake on the inside (which happens to be illegal in this country, when travelling faster than 60).
Step 4: Notice that as soon as you go past, they wake up and move over.
The only explanation that is at all reasonable is that they simply had no use for their mirrors, and were blissfully ignorant of anything not happening directly in front of them.
Incompetent, dangerous, idiots.