I’d compare it to the sobriety test of being asked to walk in a straight line. someone who’s drunk will put a lot of effort in it, which is the giveaway.
Obviously once you hit a certain point of drunkness obviously maybe a test like walking in a line can demonstrate something useful. But so would a breathalyzer. The false positives is the problem because they're being used to illegitimately subject innocent people to criminal charges.
By the time the police are commanding you to do a field sobriety test, they have already decided to charge you with DUI. There is literally nothing you can do during the test to change their mind. It's a formality, and the "test" is vague enough that the officer can cite any little twitch or misstep as "evidence" that you failed the test.
If you pass a breathalyzer test (blow under the limit), and they still want to charge you with DUI, they will likely do a field sobriety test because the results are non-numeric and are subjective. Heck, you can blow 0.0 and still get arrested for DUI[1][2], and if you tell the world about it, the police will sue you for defamation[3].
1: https://reason.com/2024/02/14/iowa-cops-arrested-a-sober-col...
2: https://www.youtube.com/watch?v=QGWSbAHaHUw
3: https://iowacapitaldispatch.com/2023/10/02/after-traffic-sto...
Just like you can also be charged and convicted of DUI even with zeros or being under the legal limit. If you’re traveling to/from/around a bar area at 2:30 AM your driving pattern and behavior is going to be heavily scrutinized.
Just because you have an anecdotal example or two doesn’t mean it’s common either. The FSTs are also completely stacked against you. Take a high-pressure scenario, less than ideal conditions (side of the road with passing cars, dark, cold/hot, precipitation, flashing lights, etc), and ridiculous/conflicting/confusing instructions and even people who are completely sober end up providing what could appear as damning evidence.
Even professional athletes have bad days where they just can’t land a shot they’ve nailed thousands of times.
I know at least a few cops who openly admit they struggle with the tests (to the point of “failure” in some cases) in no-pressure ideal classroom training environments.
Of particular curiosity is a lot of police body/dash cam footage where the officer struggles to demonstrate/explain the tests, stumbles over reading Miranda cards, etc. Evidence where if the same observations were applied to them they could be scrutinized as being “impaired”.
Of course I’m not advocating for impaired driving, just highlighting that it’s a tricky situation overall.