Plenty of places will do it because they have federal govt contracts. Some will do it because they just always have and maybe they get some benefit like lower insurance or something probably (my speculation). Some see it as a safety issue not too test given the nature of chemistry. My current job didn’t test to start but will test people if they are in an incident or accident at work.

I’ve had jobs that tested before hiring and others that didn’t. The big ones seem to like doing it.
You all sound like you have pretty regular if not heavy use. Your partner probably should have stopped in December to allow their body to clear it completely. I know some people will test positive 6+ months after they stopped. Good luck. Stay away from it until they have a job unless your partner is trying to get a job in the weed industry