Your argument is well-phrased, and theoretically correct, but in reality recruiters often work with a large subset of the job market in their area. The market is tight right now, and hiring managers have no reason to only work with one recruiter. On the contrary: they will cast the widest net, talk to any viable recruiter, and typically disclose comp information to that recruiter.

I agree it's not perfect information, but in reality if you job hop, work closely with several recruiters, and negotiate well - you'll get a pretty good idea of your market value, imho.

Employees who have the least idea of their market value are those who stayed in the same job for a long time, and haven't interviewed or talked to recruiters in years.

I completely agree with your earlier assertion that it's quite possible that entire sectors of the tech industry are underpaid, but the employers' frantic and often limited efforts to keep comp data secret are just one reason for that.