A single job posting can remain active indefinitely and does not necessarily represent just one open position.
This leads to the situation where an outsider sees one 'job posting' and thousands of applicants while Tech Co sees a shortage because only a few of those applicants are qualified.
Also, there is an incentive mis-match between people who realize there is a shortage and people wanting to prove that a shortage exists. If as a 'high quality engineer' you realize you are a scarce resource, and that makes you a lot of $, why would you want to convince other people to acquire the skills you have – self inflicted devaluation?
This is why the people with the loudest voices on this issue are CEOs and VCs who were previously high quality engineers. They have a clear view, the data, and the monetary incentive to vocalize it – more high quality engineers will increase their $.