The placebo effect is measurable. If there is no measurable improvement, there's no placebo effect either.
Bear in mind that what most claims in favour of the polygraph measure is not truth but potentially-false confession. Extracting false confessions is relatively easy, it's also completely f-ing useless to wider society and massively harmful to the victim.
You're assuming truth is the goal, which isn't correct. The goal of police is to close cases, and in this goal, polygraphs are quite effective.
They don't care if you actually did the crime, they care that they can extract a confession, or provide damning evidence to a prosecutor that lets them throw you in prison, so they can put a nice big checkmark on that case. Did they actually jail who was responsible? Maybe not, but who cares about that, apart from you?
Same reason for Forensics to exist. Don't misunderstand, some Forensic science has validity in many cases, but a lot of it is just straight up nonsense that isn't proven or peer-reviewed in the slightest, in fact many Forensic sciences that appear in modern court cases are completely, 100% debunked.
And like, why should they care? Even if you hire a crack lawyer team that gets you out of the court case, it's not like anyone involved in the investigation that almost threw an innocent man in jail is going to suffer an ounce of consequences. Or hell, even if you're wrongly convicted, worst case scenario you get a financial judgement after years of litigation, that's paid for by the taxpayers.
I see no issues with using polygraphs for hiring at intelligence agencies (I defer back to the comment about people missing the point of it), but as an investigative tool it's definitely a net negative.
The placebo effect is measurable. If there is no measurable improvement, there's no placebo effect either.
Bear in mind that what most claims in favour of the polygraph measure is not truth but potentially-false confession. Extracting false confessions is relatively easy, it's also completely f-ing useless to wider society and massively harmful to the victim.