I think a big part of this is the reluctance of people to hit and be hit. If my partner is not going to really hit me in the mouth... I will never develop the technique that stops it. If I am so afraid of being hit in the mouth, I won't stay in the pocket to develop my technique. Correcting the other guy on his lack of control, is easier than getting hit.
When I work with black belts, and we are being taught a defense from a punch... quite often I get the feeling that they are not intending to hit me. This makes me question my awesomely smooth and effective technique. So, on one of the repetitions, without informing the other guy, I just let him hit me. About 80% of the time, I feel like a Jedi from star wars... as I watch the punch go 8 inches to the left or right of my head or even stop 8 inches away. Which means that whatever dance step I was doing... it "worked." Until someone really tries to hit me that is. Many times it even takes a few tries for the other guy to actually hit me, even at a black belt level. The other thing I find amazing is that my awesomely smooth and effective technique... actually is not anywhere near as good as I thought it was. But, by working with honest punches, I can get better at the technique.
Many people train martial arts, with out actually learning how to hit someone. And they don't realize it. They can train years without realizing that they can't hit someone. They can do the mechanical movements and yell and break boards and hit bags... but hitting another person, they can't do it. Even if they think they can. (point sparring doesn't count as hitting for this point...) Think about what happens in most schools when one guy make contact with his punch during these type of drills. The most common response is "Oh! I'm so sorry, I didn't mean to hit you." And thats my point. If my partner is supposed to be working on a punch defense, then I intend to punch him. If I hit him, he did the technique wrong. (I don't intend to knock him out, or hurt him... just hit the intended target with enough force to effect the other guy, and it doesn't mean that I throw full speed either... I work at a level where the other guy can study his technique... but he is always getting hit) I check if he is okay, then we work on why he got hit.
If you train in a school where you never get hit, by an instructor who trained and never got hit... how are you supposed to know how to make your stuff work when people actually try to hit you?