Oh let's see...having a heart rate of 200, respirations at 60, standing 20 ft up a tree (most) wearing a safety harness, extra clothes, trying to shoot a live animal. That's why. You have to manage all of the things you aren't used to with only the clothing and the height being done in practice. Then it's the bow arm, pick a spot, good anchor, release clean, watch the animal as it leaves to see its reaction. I am amazed when we as bowhunters can keep all that together to execute a good shot!
It's a credit to our little group doing it without "modern" equipment, and it's damn hard controlling it all to put that arrow right where we wanted. We should celebrate the good results more because it is tough to do. Next time you have an animal down think of all the things that had to go right to hold that beast in your hands!