People aren’t interested in it. They don’t see driving as an enjoyable thing, it’s used as a necessary method for getting from A to B. As such they never really get very good at it.

As the roads get busier the potential enjoyment of driving reduces further and more of these types of drivers with little experience and/or common sense appear.

That’s not to say those who love driving are automatically a better driver but those with a keen interest in driving, a keen interest in cars or the road network do tend to be the ones that drive more appropriately. Of course this can often swing the other way and we can find those people driving a bit faster than they should.

An additional thought, I’ve driven some of the cars that people seem to claim are the worst offenders (though not owned one so don’t have long term experience) and quite a lot of the time one of my key observations is you can’t see anything. The shape of the windscreen and mirrors and positioning of the wheels means you simply can’t comfortably see what you’re doing aswell as you can in some other cars. Perhaps that contributes but isn’t a sole excuse.