Why aren't polygraph (lie detector tests) inadmissible in court?

They are not proven to be scientifically accurate.

They actually ARE admissible under certain circumstances in some courts. But they are not scientifically reliable to the degree that the results can and should be used as evidence in a criminal case. False positives are fairly common. In point of fact, the first suspect in the Sharon Tate murder failed a polygraph - even though it was later proved that the Charles Manson Family committed the crime. Sociopaths can also pass polygraphs. Likewise, there are counter measures that can be employed to screw up the results (this does not mean the person lies successfully, just that the person invalidates the test) and others are skilled enough to be able to pass.

Research has proved that some people can beat polygraph (lie detector tests). People who are able to dissociate themselves and some mentally insane people. Because it's not fact, it's inadmissible. BTW they really did ask some people if they are the President of the US or Jesus Christ and the polygraph said their telling the truth, answering yes.

because they can be duped, and there is a certain chance it will tell the result inaccuratly. Easy measures can be taken to dupe it.