Because the public is ignorant of the problems with it and the judiciary likes tools that can win easy convictions. Breathalyzers don't actually measure blood alcohol and there is no credible way they can estimate it to three significant digits but courts allow them too. Polygraphs are pure bullshit but the examiner's word that someone is "lying" is taken as sacrosanct evidence in court.

Polygraphs are not admissable IIRC.

IIRC The Wire had an accurate description of its use as an interogation tool (note I did not say investigative tool).

Not sure about DNA testing.

The other two examples are true. Across the Atlantic polygraphs are regarded as a comedy technology and breathalyser tests are sufficient grounds only for arrest and an actual blood test.

I'm pretty sure most all jurisdictions allow you to request a blood test in lieu of a breathalyser. I assumed it was common knowledge polygraphs were junk, but at least Wikipedia says they're in common use in the U.S. but are considered controversial. I knew polygraphs were banned as a prerequisite for jobs in many areas and that many government jobs had them as part of the interview, but figured it was more of a stress test or hazing. I do think the bar for expert testimony and scientific evidence could use a lot of improvement in the U.S.

In the UK there used to be such a right to refuse the breath test and take a blood test instead. However it was removed.

https://www.theguardian.com/society/2009/mar/15/drink-drivin...

What the actual fuck? I didn't realise that the UK was so stupid. Is due process dead here?

Polygraphs aren't admissible on this side of the Atlantic as evidence.

I don't know if this is the commenter's intent but isn't there a potential false positive issue with DNA testing?

Something along the lines of if only 0.01 percent of the world's population can match a DNA sequence found that still leaves us with a million matches where I just made up both those numbers.