I don't know

I don't know why people are so reluctant to say that they don't know something when they don't know. When I think about it, I don't understand why they are so scared or ashamed to say that they don't know something.
I feel its perfefctly ok to say "I don't know" when I dont know. I don't like to beat around the bush like how many people do. I dont feel ashamed to say "I don't know". I can't be expected to know everything in this World. There are a lot of unknowns in this World for me. I had one such incident during one of my interviews long time back, It was a technical interview and the interviewer was asking all kind of questions. I answered some of questions for which I knew the answer, for the rest I told him directly that I don't know. So this guy asked me, How I could say I don't know in an interview. I told him that I am learning from my experience and I haven't worked in the area that he was questioning and hence I could not answer his questions. Well I did'nt get that job. But I felt that I did the right thing.
I have also come across people who alter their resume as per the requirement advertisement. They say that they know a lot of things that they actually don't. I feel its completely wrong to do that way. Why not say "I don't know" when they don't know?
I also believe that unless we accept that we don't know, we can't learn. If X beleive s that he knows when he does'nt know then I am pretty sure that X won't be putting the effort to learn what ever he does not know.
When I searched the net I came across some interesting analysis on this subject. Most of us think that we either know something or we don't know something. But the author says that the more acurate way to look at this is

You can either know or not know something, and you can also know or not know whether you know or not.

As per the author, knowledge can be split into four quadrants with "what I know" as a dimension and "awareness of my knowledge" as the second dimension.
The best quadrant to be is in the first quadrant, To know that you know. If not in that quadrant then it is better to be in the 3rd quadrant, where you know that you don't know so you can learn what you don't know. You become teachable when you are in this quadrant. When you don't know what you know, you just wasting your knowledge.
When you are in the fourth quadrant, where you don't know that you don't know, this is when the communication becomes a problem. A person in this quadrant become self-proclaimed experts, ready to share their opinions to anybody ready to listen and thus refuse to listen or learn from others.

(Source: STSC)

Varta...