They’re not admissible because they’re utterly unreliable and mean nothing. Pretty much the only thing they’re actually good for is playing the “bad cop” part in good cop/ bad cop and getting people to confess. The whole science they’re based on is utter bunk. The damning part of this whole story is that the FBI still uses this junk science on its own agents.
Even worse, being a polygrapher is a "profession" with its own lobbying associations, the American Polygraphy Association and National Association of Polygraphers.