Here are some reasons why polygraphs are not considered reliable:

- Physiological responses can be caused by factors other than deception. For example, someone may experience anxiety or physiological changes due to stress, fear, or excitement, even if they are not lying.

- People can be trained to control their physiological responses. This means that someone who is skilled in deception may be able to "beat" the polygraph by controlling their breathing, heart rate, and other physical responses.

- Polygraph results can be influenced by the way the questions are asked. If the questions are leading or suggestive, they can increase the likelihood of a false positive (indicating that someone is lying when they are not).

- Polygraph examiners can make subjective judgments. The interpretation of polygraph results is often subjective, which means that different examiners may come to different conclusions about the same person.

- Polygraph results are not admissible in court in many jurisdictions. This is because polygraphs are not considered to be reliable enough to be used as evidence in a court of law.

Despite these limitations, polygraphs are still sometimes used in law enforcement and security settings as a screening tool to identify potential suspects or to gather additional information. However, it is important to remember that polygraphs are not a reliable lie detector and should not be used as the sole basis for making decisions about someone's guilt or innocence.