I have yet to understand the historical underpinnings of this phenomenon, but as someone who deals with chronic pain, doctors don’t seem to be aware that people feel pain _in general_.
I’m being sort of facetious, but any chronic pain community is full to the brim with stories of various medical professionals being skeptical of patients’ pain reports or treating pain like it’s not a particularly important quality of life concern.
I have my own stories about this, of course, and some of them are pretty horrible.
It doesn’t surprise me, then, that we assumed at some point that infants don’t feel pain: after all, if we don’t take the pain of adult humans seriously, then why would we consider the pain of a creature who cannot even directly complain about it?