Why don't the court use a lie detector?

Because they want to prove a case "beyond a reasonable doubt." So since lie detector tests aren't 100%, then there is no way to use them and prove "beyond a reasonable doubt"

It's complete pseudoscience. They don't actually work. How do you think Aldrich Ames managed to beat all of them while spying on the CIA?

it's easy to alter the results but tensing your body and stuff, they're nowhere near accurate enough to stand up in court

You answered your own question. It's not 100% accurate.

Because you are better off flipping a coin to decide if the person is telling the truth or not.

It's not even close to accurate. I took a test and the thing registered a truthful response to my currently living on Mars. They're easy to beat.

because its based on how nervous you are and many people who have been arrested are nervous which will effect the results

Because polygraph machines don't detect lies at all. They measure various physiological signals that indicate increased levels of stress and nervousness, which MAY (or MAY NOT) mean that the subject is lying. If polygraph machines were accurate enough to use in court, then criminal trials wouldn't exist in the first place.

Exactly because it is not 100% accurate. It is pseudo-science and no better than a 50-50 guess. It may make good entertainment on the Jeremy Kyle show, but any court determining peoples liberty based on hocus pocus risks becoming a laughing stock.