Why do lorries take so long overtaking each other on motorways?

Because if one is travelling at 66 miles an hour and the other one is travelling at 66.5 mph it will take the one going faster an hour to move 1/2 a mile ahead of the one that it is overtaking. Where as in a car you could be travelling at 10-20mph faster than the lorry without really trying (obviously you shouldn't because you would be speeding) but most of us do.