My guess is that it's mainly because postdocs are an unknown quantity without legible, interpretable experience, which greatly increases the risk of hiring them

My guess is that it's mainly because postdocs are an unknown quantity without legible, interpretable experience, which greatly increases the risk of hiring them. One of the main achievements of the Insight Data Science program (which I have participated in), is that it makes you produce output (a web app) that is legible to employers. People are very risk-averse when making hiring decisions, and tend to hire people who know what they know.