Why do they teach such complicated and difficult techniques (bunkai) in karate?

Well, it appears it has something to do with perceptions and the brain. It appears that the more options, the more information and the more difficult it is to either do something, learn something or apply something that the brain and our perceptions say to us, "It is important." If it takes more of our time and more of our attention it has greater worth therefore it is more important.

Our brains are programmed at an unconscious level that which is more difficult, any type of difficulty, is important and important means it must be true, it must be applicable and it must be the better of our choices.

A good example is part of an explanation given at the redtape chronicles article today that the plathora of options for products provided today create a mind-set that to choose one is most difficult causing us to go into a mode of, "this is very important so I must take time to choose wisely," along with a lack of information resulting in our "difficulty" in making a choice. He calls it "decision quicksand" which inspired this post on the "difficulty of applying complex and difficult techniques in application, both kumite and survival on the street. It is that difficulty that causes the mind to go into importance and difficulty mode resulting in "pauses or freezes" of a kind.

No wonder most professionals teaching survival techniques teach is that the simpler and the more closely associated with movement instinctive to said survival the better the technique to apply for street survival. I know from my limited personal experience that the simpler techniques worked best for me.

Think of it, do your rank tests seem difficult and complicated? If yes, are they truly indicative of survival applications of said techniques? Is it possible that unconsciously those setting up requirements and testing naturally know that the more complex, the more difficult that the perception is that it must be good, important and applicable to what a person is doing in that particular martial system?

Questions on exams, not martial system type but normal school stuff, can be either simple or difficult. If you take one that is simple you feel like it is a waste of your time but if those same questions are then formed properly they tend to appear difficult so you walk away feeling like you "did something important."