Just not weeks after consumption...
My point is that the test is testing the wrong thing. A sobriety test would be best.
Doesn't that say something about the effectiveness of testing?
Culturally, recreational drug use isn't that big of a deal, at least to millennials. You can completely be responsible enough to enjoy some cannabis on personal time, while also doing your 40 hours productively.
The problem is one of information from the POV of the employer: they are ultimately responsible for what the worker does on the job.
Even if P(no problem | drug use) = 0.99, the amount of negligent liability they will incur if an accident does occur, the person was intoxicated, and the employer could have checked for drugs, but didn't, is probably so high that they can't take that route.
While I agree that drug tests are a scam as a white collar worker, I do think they're necessary from a liability perspective if you're in construction, manufacturing, or driving a company vehicle.
I fully support the use of recreational drugs on someone's own time, but you have to weed out (forgive the pun) those who are under the influence when loss of life or limb is a risk.