There's also the fact that in order to support a Starfleet the size of which we've seen, they need more than one "academy" to create that many officers. Yet we only ever hear of the academy in San Francisco. There are a few smaller schools, but they seem to be annexes rather than separate schools.
There have been other Starfleet academies in the novels going back to the 80's.
Do we know how large the Academy is?
It never quite looks the same or stays in the same place.
Here's a totally over-the-top analysis of Starfleet and the academy's ever-changing appearence and location, with maps!: http://www.ex-astris-scientia.org/in..._buildings.htm