I’m not defending it, but it’s true - and on today’s motorways, everyone else is basically doing the same anyway - sitting there on ACC mode.
Until more cars come with auto lane change built into their level 2/3 driver assistance systems, many folk will do this.
I passed my IAM back in 2008, I can drive at top tier level, I just can’t be bothered to when the computer will do an acceptable job

And to be honest, most of the time in lane 2 you are overtaking trucks, so it’s still legitimate to sit there at 70.
Then there’s this average speed zone rubbish where overtaking just can’t happen anyway, and everyone is bunched up at the same speed for “safety” (in reality, the opposite).
I wouldn’t say it’s “disengage brain” either. You can actually pay more attention to the traffic when lane and distance keeping are handled for you by a machine.