Rat 5 is bang on the money..
I'll give a practical example of one of his points.
Many of the younger/newer pilots that I have flown with don't run a 3x profile in their head during descent. To those of us who used to have to run it constantly all the way to touch down this seems absurd. But they have never had to, and nobody ever trains them to do it, so they don't, they just check that the VNAV pip is in the right place and then feel comfortable.
So, as Rat pointed out, when the profile is wrong, when do they pick it up? Passing 20,000ft when they expect to have 70nm to run but only have 55? No, they don't even notice because they have never been controlling it manually. Scary I know but it's true in many cases.