They fill a similar role to prosecutors that field drug tests serve to cops[1]: they aren't meant to be accurate, they exist to give the cops legitimacy as they proceed to do whatever it is they wanted to do to you. In the cops' case, they need legitimacy to search and arrest you, and in the prosecutors' case, they need legitimacy to charge and convict you.
If field drug tests were actually accurate, they wouldn't be bought and used. Similarly, if an expert witness fails to give prosecutors the results and testimony that they want, they wouldn't be hired again.
If field drug tests were actually accurate, they wouldn't be bought and used. Similarly, if an expert witness fails to give prosecutors the results and testimony that they want, they wouldn't be hired again.
[1] "How a $2 Roadside Drug Test Sends Innocent People to Jail": https://www.nytimes.com/2016/07/10/magazine/how-a-2-roadside...