It’s not difficult to demonstrate that IQ is relevant to job performance (its the best predictor of performance for most jobs). The problem is to demonstrate that it doesn’t have a discriminatory effect (there are large, persistent differences in IQ between races).
There's already an imbalance when it comes to different genders and races, I don't get what they would lose.
The real reason companies don't do it is because Fizz Buzz is a better test than IQ tests.