I understand that, but I don't see what is so complicated about reporting # of HIV tests conducted in a certain timeframe (daily, monthly, whatever) and what po

I understand that, but I don't see what is so complicated about reporting # of HIV tests conducted in a certain timeframe (daily, monthly, whatever) and what portion of those tests came back positive, just like we currently do with coronavirus. It doesn't matter what the lag time is in between being exposed and receiving a positive test. What we are interested in is # of positive tests, why is there a 3 year lag here?