They collude, more or less. Although indirectly via third parties (such as Radford[1]) so it stays legally kosher.

"Neutral" third parties conduct salary/compensation surveys. Many companies (HR departments) participate in these surveys and disclose their information to the third party.

The third party then packages it up into an anonymized 'market compensation analysis' that companies can subscribe to/purchase.

So companies can get a good read on how much is enough for their market, without ever directly revealing their hand to anyone except for the neutral middleman that's washing all the data.

A lot of HRIS (HR Information System) companies and payroll companies also package up and sell aggregate analytics on compensation figures. And the companies using those vendors are pretty complicit about their employee data being used in this manner, because at the end of the day they're the same ones buying the end result and peeking at the aggregate data to ensure they don't offer any more or less than they have to for a role.

[1] https://radford.aon.com/

There are organizations that try to collect this data, I know my company currently uses one of them to determine what a competitive wage would be for hiring. I forget what they are called though, and their data does seem to be in favor of paying less (maybe outdated data, maybe trying to side with the employer, who is their customer, that they don't need to increase pay, idk).