What's the Deal with Motorways and Lane Hogging?

Coming back from a family event yesterday down the M40 and the amount of lane hogging was insane - are these people allergic to the left hand lane or something? Had one person sitting in the middle lane for miles doing 65ish, but when I started passing on the left lane (I was doing 70 and had been in the left lane for a good mile or so), they'd speed up and end up just sitting in my blindspot until I slowed down.

Seriously, why does this happen on motorways so often - I also see it all the time on the M25? Is it just a lack of understanding or inexperience driving on them? A complete lack of awareness at how much congestion it causes? I never touched a motorway during any of my driving lessons, but what is it about motorways that make people forget "use the left lane unless overtaking"?