O
Because it is like practicing bleeding. Thirty- forty years ago a friend and I went through ground jump training followed by a jump. Having just had several surgeries on my left foot, I opted not to jump. My friend jumped and ended up with a broken ankle. I lost track of him after about 5 years but he was still walking with a limp.

My favorite story about this was a friend who had lots of skydiver friends and was a glider pilot so he thought he should jump for training. As he was standing in the door about to jump, he turned around to yell to his friends "This is crazy, it has to work the first time anyway!"...