I'd offer a different take: they are required often for skilled manufacturing jobs, like for people operating precision or dangerous machinery.

I've seen many anecdotes where factory owners say that they can't find workers for machine operator positions who don't test positive for meth or opioids.

It's also possible that some of those employers are using drug tests as a sort of "morality" bar, but I'd bet they primarily want to protect their expensive capital equipment and their other non drug using employees from potential drug-use related accidents.

But I doubt there is a lot of drug testing for burger flipping or grocery bagging type jobs, especially not in a tight labor market as exists today.