I think it's a mixture of carelessness (i.e. intentionally speeding without knowledge or regard for the possible consequences, which I see in the city a lot), and modern diesel cars. I would never have considered the latter until I owned one for a short time a few years back (2.0 165ps CDTi).
Unlike petrols, a diesel will generally surge ahead under its own torque. Moving back to petrol was a shock because unless you hit the throttle it won't move, and the second you ease off the forward momentum stops. Not so in a diesel. Add in the insulated detached modern cabin, with no real feel or connection to the road, and then an engine with a monotonously linear power delivery.... It honestly doesn't feel much different doing 90mph as it does doing 20mph - the scenery just moves a bit quicker if you look to the sides of the car. There's no feel of acceleration or speed at all. Boring as hell, but also easy to imagine how someone will end up driving faster than they should.
For town and city driving I'm with you OP. The first thing I got taught doing advanced driving was to (almost) always obey red ringed limits. If you're paying proper attention in hazard rich environments like towns (scanning every junction on approach, checking cross views, looking over-under-round-and-through every parked car for movement, watching every pedestrian etc) then you'll find you naturally can't go very fast anyway as the speed falls off with the increased obs. Once the derestricted signs appear, all bets are off.
As for the mindless morons, I get the usual abuse for overtaking on an almost daily basis (we have a lot of countryside around us, but populated by lots of clueless city drivers). I had a cracker the other week, on a two lane dual carriageway. Bear in mind I have a 2.0 TSI 220ps, but I was low on petrol and cruising the 10 miles to my mum's (who lives near a Costco selling 99 RON super for the price of supermarket 95 RON). I figured I'd cruise gently to hers, with 20 miles of range, fill up and all would be well. In other words, I'm certainly not hooning it lest I break down half way there!
I'm in lane one, and the traffic lights ahead go red. I'm at the stop line, first car in the 'queue'. A young girl pulls up next to me in lane 2, in her Peugeot 207. The lights go green and I (very) gently accelerate away up the steep hill ahead. Despite being careful I pull ahead after a few seconds (220ps versus about 60ps don't forget). Does Peugeot girl put her foot down? No, she slowly drops back, pulls in behind me, and spends the next few minutes flashing her main beams and beeping at me. W. T. F.?! Was I supposed to 'let her win'? She'd have had a real shock if I wasn't trying to save fuel haha...
). 30 seconds later there's an NSL, and I invariably catch up to them and end up stuck behind them as they continue at the 40 mph they've been doing since exiting their driveway... Before the road turns into a dual carriageway and they continue doing 40 in the right hand lane because they're turning right at the roundabout in a mile 
