There have been comments of around 12 secs of useful consciousness after an explosive decompression. Ignoring all the other chaos in the cockpit for a moment ( noise, debris, panic, cold etc ) why would pilots lose consciousness so quickly when people can swim underwater for much longer than this.
Because air pressure in their lungs is almost (to be exactly very slightly above) environment pressure. IOW. as decompression occurs air leaves their lungs too.
People holding their breath under water hold onto oxygenated air in their lungs. You can't (and shouldn't even try - if you'd forcibly try, your lungs would rupture and you'd be in even worse shape) to hold air while in decompression.