Lie detectors don't work
Lie detectors are not accurate. They are little better than a coin toss.
Many legal systems operate on the founding principles of a trial by jury. In these cases, the defendant is entitled to be tried in front of a jury of their peers. This jury will be the sole body that determines guilt beyond reasonable doubt.
Lie detectors, by nature, undermine the fundamental principles of a trial by jury. They diminish the jury’s role in determining guilt. The jury would likely give too much credence to a lie detector result and overlook other strands of evidence presented.
Assuming that a jury can’t understand that lie detectors are not infallible and would not use them to supplement a decision but as the basis of the legal decision undermines the intelligence of the average citizen.  Juries are capable of understanding that lie detectors should be used as a supplement, not the basis, for a case. They can be trusted to carefully consider the evidence presented, of which a polygraph test is only one, potentially fallible part.
[P1] The jury is assigned the task of determining guilt, not a lie detector. [P2] If lie detector tests were admissible in court, they would undermine the principles of a trial by jury. [P3] Therefore, they should not be admissible in court.
[Rejecting P1] The jury would still be in charge of determining guilt. [Rejecting P2] The polygraph test results would just be another piece of evidence. It would not form the basis of a guilty verdict, but nor would it be dismissed. [Rejecting P3] Lie detectors are a useful tool for establishing deceit. Therefore, they should be used, alongside other legal tools to measure deceit, and be admissible in the court of law.