Lack of testing, for starters. There was a time, going on 40 years ago, that someone observed my driving and decided whether I was safe to release upon the roads. Since then, I've taken a knowledge ("written", though it's computerized now) now and then. The last time was 16 years ago when I moved to WA. They check your vision at renewal by testing your ability to check a box that says "yeah, my eyes are fine".
Add to that a lack of enforcement. As long as you don't exceed the speed limit or have booze on your breath at a checkpoint, you might go your whole life having never been pulled over no matter how much texting you do.
Here in Seattle, which I maintain has the worst drivers in the country, my current working theory of the cause of horrific driving incompetence is self-absorption that seems to be particularly rampant here. Why else would I get regularly flipped off when the light turns green and I honk at Ms. Tweets McFacebook to put her phone down and drive?
I've driven quite a few times in different parts of America and the lack of courtesy is frankly ridiculous and is almost an extension of that "I've got mine, fuck everyone else" attitude that some (not all) Americans display.
I have to say as another Brit, some of the cars I've seen on the roads of America wouldn't be allowed near a carriageway here. It's not just the standard of driving that is sketchy, but the maintenance or lack thereof too. I've driven on roads that felt way more iffy that anywhere I've been in the third world.
It's an unfortunate situation and not one I agree with, but that's why American drivers tend to suck.
I didn't try driving in London though.
Motorway driving is not part of our driving test, so you can qualify without ever being shown how to drive on a 3 lane road.
It's not uncommon to see people cruising along in the middle lane, completely oblivious. Drivers are noticeably better at keeping right in continental Europe.
Not that UK drivers are perfect (very far from it). But I've heard the same comments enough times to make me wonder how American drivers are so bad.