And it's not purely a social problem; it starts in the Rules of the Road, which says things like:
You must progress at a speed and in a way that avoids interference with other motorway traffic.
Avoid driving too slowly
In normal road and traffic conditions, keep up with the pace of the traffic flow while obeying the speed limit. While you must keep a safe distance away from the vehicle in front, you should not drive so slowly that your vehicle unnecessarily blocks other road users. If you drive too slowly, you risk frustrating other drivers, which could lead to dangerous overtaking.
While it does remind you to obey the speed limit, this comes into conflict with the greater message here: "keep up with the pace". Since it seems like the majority of other road users are almost constantly speeding, this reinforces the notion that you should never travel below the speed limit, and if anything, increase speed to match traffic in front.
It's frustrating - if you're driving within a city, given the time spent in traffic or stopped at lights and the relatively short distance, it doesn't really matter if you travel at 45 or 55. But it's considered a greater crime to err on the low side. Why?