Medicine isn't nearly so scientific as you probably think. An average doctor might see a weird case once or twice that was actually AIDS but that's hard to separate from all the other weird cases they see on a daily basis (House: maybe it's lupus!). In the 70s there certainly weren't any good central databases for general medical records, and there still aren't, especially in the US, because of privacy and insurance concerns.
If you were a doctor in the 70s and you saw a malnourished person waste away and die, would you think "gee, it's horrible we let people starve on the street in America" or "OMG, this is the start of a plague that will sweep the world in twenty years"?
In the early 80s, when the number of patients increased, doctors, especially those who worked in gay communities, who were most at risk, DID notice unusual numbers of people dying and did report and track it.