As Nige F and T.C mention its the inability to read the road ahead that is the main problem, its not taught on either bike or car lessons & not even mentioned by a lot of instructors
When I was bike instructing & doing post test lessons if I started to talk about the apex, road positioning, vanishing point or anything to do with assessing a bend or the road ahead it was like I had started to talk another language to some people, those that had been driving a car for a few years previous to taking the bike test were generally the worst
The other thing I have noticed over the years is some people simply don't have the capacity to process all the visual information very fast so consequently end up entering a bend quicker than their brain can work out what they need to do next, as T.C mentions when this happens the old adage of slow in fast out goes right out of the window (we have all done it learning from it is the key)
In the end I got pretty sick of trying to get people to look up & assess the road ahead, I would say less than 30% got it & went on to become good safe riders, most of the rest seemed to get by & no doubt it will have clicked for some later on but a small percentage will have never got it because they knew best, I just hope they survived
Funnily enough it always seemed easier to teach women, they always seemed to genuinely want to learn, only a few years ago I had a bloke who's wife I had taught from scratch having never rode a bike at all before telling me how she had become a great quick & smooth rider over the years since passing her test, made me really quite proud that did
His next comments sum it up for me "she's really good in the bends" and " I don't know how you did it, what is the secret?"
it took all of my willpower not to tell him to be honest "secret" my arse, he had be riding 30 odd years but wasn't willing or able to learn a damn thing