I suspect the bigger reason is because Ph.Ds are difficult to accurately assess. When a company wants to hire a candidate, they give preference to someone who h

I suspect the bigger reason is because Ph.Ds are difficult to accurately assess. When a company wants to hire a candidate, they give preference to someone who has a connection to the company or to someone on the inside. Ph.Ds are extremely unlikely to have any meaningful connections in this regard. This makes vetting them difficult as there is no one to vouch for them. Location of papers is a decent metric, but more and more it's an indicator of whether the Ph.D comes from an 'in lab' in an 'in field'. Quality of publication would be the best metric, but that is extremely difficult for an outsider to judge. This leads to a shortage as there is a shortage of people a company can reliably hire at the price they want with a skill set that justifies the price.