Because it is SCIENTIFCALLY proven they are BS.
They have been proven unreliable under some circumstance. The last rating I heard was 97%(still you know about ratings, they are only as good as the person giving them). They do have problems. If you test someone who is delusional, believing their own lies, they will pass. If you test someone who has forgotten things, or has mentally buried things, then they will more than likely pass the test. If you happen to be a CIA agent, someone who has been trained to fool the machine(which is harder now than 20 years ago, but some people can do, especially if the have NO doubts that they can). or if they don't believe it will work, their breathing, sweating, and other signs may not vary enough to get an answer.
As for being nervous about a test throwing it off, don't worry about that. Everyone normal will be worried about a polygraph test. You will be ask quite a few questions, which the examiner knows are true or false, and a few he/she is not sure of, just to help gauge the test. Once they get a baseline for you, you will be just as nervous for the true questions as well as the lies, and the lies will show up beyond nervousness.
The problem (If I remember this lecture from UNC) is when they ask you a question like, "Are you guilty of the murder of Adam Bell?" You answer no, because you did not kill him but you remember not warning him about Steve being angry at him and finding out about his affair with Steve's wife. You feel guilty about the death, and the test shows an inconclusive result. If you feel guilty enough it will show a deceptive answer. A deceptive answer does not mean to the FBI(even though they may tell you different to squeeze the "truth" out of you) that you lied, but that you have something going on emotionally with that answer.
These tests are not perfect, and cannot be used in a court of law, but can be a tool for LE to help them know where to search, and where not to search.
