I have always wondered about this. We all know the system is complex, but what is it that makes it so difficult?
Is it the number of plays in the playbook? The reads the QB is required to make? The pre-snap reads and audibles required of the QB? The option routes the WRs have to run? All of the above?
I have read a bunch of opinions that included some of the reasons listed above, but I don't think I have come across a definitive answer.