This statement is proportionally more common in fields where it's challenging to sort truth from bullshit.

If it's easy to see that a piece of output (a paper, code library, machine learning model, whatever) or a job candidate is great, then the credentials behind it don't matter much. However if it's challenging to evaluate quality, then people will shift to looking at secondary signals such as credentials, price, etc.

As to why a lot of economists go with the signaling model of education, well, it might just say something about their field and how much they got out of their own educations.