Hmm, I don't think that's true. I score incredibly highly on the 'verbal IQ' segment of the test, and anecdotally I seem to give people the impression of intelligence, but in the mathematical segment of the test I score about as highly as a potted plant.
Unless you've ever independently (non-circularly) corroborated your 'idea of how smart someone is', my guess is that it just serves as a proxy for qualities like academic education (which != intelligence), social class, similarity to yourself, &c.
It's a small market with a few big very successful companies leading the way and then the smaller companies imitating as many of their practices as possible. In Sweden the biggest one of them would be Spotify and Klarna both have had IQ tests as part of standard hiring for some time, not so sure Spotify still do but definitely a few years back and there's a lag before the medium sized companies pick up on the changes.
For the record, hiring is hard, no company seems to have got it right. Think IQ tests probably says more than someone being forced to nervously scribble down bubble sort on blackboard or estimating how many water melons there are in pakistan. None of these obviously says anything compared to working with someone for a week.
edit: People are different, if it was as simple as evaluating a conversation it wouldn't be a very hard problem.
Unless you've ever independently (non-circularly) corroborated your 'idea of how smart someone is', my guess is that it just serves as a proxy for qualities like academic education (which != intelligence), social class, similarity to yourself, &c.