This is one of the issues with drug testing. With alcohol, you drink it, it is in your system and impaired, and then it leaves your system. It makes it very easy to test for since if it is in your system, you have taken it recently and it is currently effecting you. With other drugs (pot being the most obvious) there is no simple way to test for it. It stays in your system so long there isn't a way to tell whether you just smoked a joint and are impaired or you smoked a joint last night and are fine now.
This is one of the things that is being skipped over in the rush to legalize pot (I'm not saying legalization is a bad thing, but not everything is being handled). There needs to be a way to tell if people are currently impaired as we don't want people driving/operating crane/whatever while they are high. Currently there isn't a good, standard, scientific way to do that (and yes, I know breathalyzers have plenty of problems as well). Until that happens, the best we have is either blood tests which show how much THC is in the blood, but not if it is actually affecting people or sobriety tests which can be highly subjective.