Because belief is not rational. The people that passed the polygraph tests and administer them to others may acknowledge the flaws but they still 'know' that a lie detector is the best they have got. They also know that some people will just confess at the thought of being wired up to such a machine, to give evidence that, unlike the lie detector, is admissible in court.
The most interesting story is how the authorities nabbed the guy who was most dedicated to dismissing the lie-detector baloney:
https://www.theguardian.com/us-news/2015/may/12/polygraph-cr...
His consulting work of helping people pass lie detectors was his downfall. Had he just stuck to campaigning against the lie detector and not wanting to offer services to those needing to pass a lie detector test then all would have been okay for him and he would not be having to serve one of those silly 100 year sentences that American justice demands.