This is my own theory / thought, I might be wrong : logically thinking - if you wrap metal to keep in that sort of high levels of heat, the metal *might* warp or crack under pressure. Or it could be that the heat that is supposed to escape through the extractor doesn't dissipate fast enough and the temperature in the block increases and eventually gives way and cracks?
I've never actually seen a cracked extractor so I have no idea. What I do know is those track / racing cars wrap their headers all the way to the exhaust. But racing cars are always rebuilt before each race - can you afford to rebuild your car everytime?