Why don't you ask a civil surgeon why they don't do it and post their answer here?
I can see why they don't want to do forms with missing parts which can be filled with anything later... So they want to do the form as a whole. If there were different forms for vaccinations and medical exam, I think they'd be more ready to sign only the vaccination part.
This question gets asked periodically on VJ. I'm just saying what I observed. Maybe signing off only vaccinations doesn't make financial sense to medical professionals. If the entire exam is $500, they won't be charging a small amount for smaller job, it's not worth their time.