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?
Even worse reliance on automatics and VNAV: when adjusting the route during arrival; perhaps with an extended centre line when still flying towards the downwind and needing to make a 180 to the runway, the VNAV vertical error might give >2000' high. What happens is the new generation yanks out the speed brake and dives the a/c to attain 'the path'.
Ask them to look at the ILS DME = direct track miles to the airfield; now add 15nm for the downwind, base turn and finals from 2000'. Are you high or low? Very often the answer is LOW. There is an ah ha moment, the speed brake is retracted and now the question. "How can VNAV show that error?" "It doesn't matter, but it did. Now do you understand that you are in charge and the FMC is a tool to help you, but you have to decide if you trust the info it's giving you and whether to use that info." Your brain needs to be further ahead than the nose of the CDU.
It's like the guys who punch numbers into a calculator with no gross error cross check of the answer expected. They too have some surprises.