I think you're underestimating some of these kids confidence. No doubt there are a lot of shy kids as you would expect, or guys who don't want to risk losing a spot.
But you also get the damn good players who know they will be picked up (matter of where in the draft, not if), have been told they are damn good since a young age, are intelligent, and are popular among their peers - these kids are up to their eyeballs in confidence and self-esteem and if some coach was rude to them/challenged them, they would go straight back. Brodie Grundy seems to be an example in this thread. These kids exist. Replying in kind is not necessarily being a dickhead.
Only if you're a gun and you KNOW you'll be picked up based on your talent and level of interest. Those on the fringe and 'hopeful' of being drafted wouldnt do it.