I'm also pretty sure most government jobs require it periodically.
That also makes it kind of obvious that someone is getting ready to jump ship if they stop smoking marijuana for a couple weeks.
If you work in a safety critical, or security critical role, then there is randomized testing as well as pre-employment testing. This can be especially frustrating if you work as a contractor in those industries...
For driving/piloting jobs, they might add alcohol, but only for a spot test whilst on duty, not general consumption.
A lot of our issues stem from our (mostly) Republican Party's "War on Drugs". I could give you a rundown, but the summary section from Wikipedia does a good job.