I can assure you that they really were a thing. We used to joke that they were "pimps."

But I worked for one company for about 27 years, and never used one (except when hiring).

Back in the late 90s it was definitely a thing. I'd argue that part of what diminished it then was the permatemp lawsuits over who was an employee vs contractor. Many BigTech companies swung away from working with independent contractors for a period of time.

Same. I would be way more interested in contract work if I didn't have to do all the work to find clients. I would trust a human being more than a website whose only goal is to lower my value while raising theirs (ex. Upwork).

Yes. I dabbled with one a couple of decades ago. I didn't find it to be all that fruitful of a relationship, though, which may be why they have fallen out of favour.