Clandestino
Originally Posted by Ian W
(...) it is because the human brain has a limited number of cognitive channels and they can only handle ONE input at a time.
That's the best explanation of someone's inability to chew gum and simultaneously tie his laces I've heard in a long time. Attention distribution, proper scanning and avoiding the fixation are basic pilot's survival tools, taught and checked from day one at the controls. If you suck at them, tough luck, you can not be a pilot. What AF447 crew did was the sign of the severe incapacitation, not usual and ordinary human behaviour.
Each of the cognitive channels in YOUR brain can only handle one input at a time. There are discussions about how many cognitive channels there are but if you really think you can concurrently read a text, while repeating a well known rhyme correctly, listening to someone talking and accurately writing down what they are saying - then you are NOT human.
I suggest that you google 'Wickens cognitive channel' and read the research there much of which was carried out with pilots. You will see how easy it is to initiate cognitive tunneling in professional pilots.
It is the poor designs and procedures that do not take account of human cognitive limitations that can lead to accidents.