Why don't demos translate into real fighting?

I was watching some ISKA forms, sparring, weapons and breaking competition on cable tv the other night. Why does that kind of fighting just not really work, say, in the UFC? If a guy can shatter a coconut, why can't he hit a UFC fighter in the head and knock him out? Conversely, i will bet that Matt Hughes can't break a lot of boards, yet those guys likely would tap out against him in a few seconds. I'll bet Randy Couture would look awkward trying to do a kata, and likely can't even throw multiple kicks, but could probably run through each of those guys on the same day. This has never made sense to me. I saw a short 55 year old karate master shove his thumb through a brick, but Jon Jones would likely kill him in the ring. Confusing.