Perhaps because they are not held to the same standards as an MD. PAs are typically employees in a clinic, hospital, or practice, and they work with limited autonomy under the supervision of MDs.
I'm not sure I'd say that PAs are not "professionals" but I would say that they are only in a broader sense of the word.
I'm not sure I'd say that PAs are not "professionals" but I would say that they are only in a broader sense of the word.