Why do you think that's the case? A typical company is hiring at least one person per month, and depending on the size of the company, probably many more. Plus, if people keep telling them the numbers, they can get a pretty good idea of the state of the market just from talking to all the candidates, which gives them quite a lot of datapoints. Do you do a monthly salary survey of a random selection of 30 people in comparable job roles?

I think it goes both ways. The employer may have talked to 30 people looking for comparable job roles in the last month but they almost certainly haven't ever talked to the 10 other companies in the area that are hiring for similar job roles.