My understanding is that it's an open secret that they're bullshit. Slowly this knowledge has percolated into the culture, which is why you no longer see them in media any more, it's not seen as realistic anymore.

The premise of the classic lie detector--the one drawing that line back and forth on a piece of paper--is that people become slightly agitated when they tell a lie; their body goes into a fear response at being found out.

Well, it turns out it's incredibly stressful being interrogated in general hahaha and there's actually no way to distinguish between 'oh shit i hope they don't fuck me up' and 'oh shit I hope they don't realize I'm lying and fuck me up' hahaha.

Even the more advanced ones based on pupil dilation and electro-conductivity in the skin are useless for that simple reason. They are not acceptable as evidence, as far as I know.

Indeed, police have known about this for a long, long time, and actually have used lie detectors for show. A lot of people believe they work, and so are more likely to be forthright if they think the police can use this contraption and suddenly see right through them.

So, don't worry, they're a glorified intimidation device at best, which doesn't work if you can see right through them ;)

Edit: Apparently 'they' have developed a new lie detector, which measures specific neural pathways in the brain, and can apparently distinguish a truth statement from a lie. But it's yet to have been tested in the real world :/ I... don't look forward to that era, should it come.