* Source of the Hair: Follicle drug tests analyze the hair strand for traces of drugs metabolized and incorporated into the hair as it grows. A wig, even if made from human hair, was not grown on the person being tested and therefore will not contain their personal drug history.
* Lack of Personal Metabolism: A wig hasn't been subjected to the individual's metabolic processes. It doesn't carry traces of anything they've ingested or been exposed to.
In short, a wig is a foreign object and will not reflect the test subject's drug use.