They don't cover CPS in medical school. And just because it may seem like common sense/knowledge to *us* that rear-facing, for as long as possible, is safest, it may not be to them. Their brains aren't wired that way. They aren't going to run through physics and crash dynamics and skeletal structure before covering something that was pre-set for them to cover. They're just recommending...what they're "supposed" to be recommending in that departmemt. No more, no less. Not because they don't care, necessarily, but just because it's not their area of expertise.
I compare it frequent times to vaccinations. Many (not all, but many) pediatricians can't tell you what, exactly, is in a vaccine, or how it's made, or when the last case of wild polio was, or a real, true run-down of risk v benefit. They just know why they exist and what is it they're supposed to follow and recommend.