(The cynical side of me says it's because they give superiors and judges a reason to pass their opinion as judgement on someone without any real evidence...)
Give your cynical side a cookie. Now I might be completely wrong here, but the above is really the only reason I can fathom, either that or complete and utter ignorance... Which of course isn't too far-fetched an option... Luckily, polygraphs are not used in court over here in Sweden.
"Atomic batteries to power, turbines to speed." -- Robin, The Boy Wonder