I’m pretty sure they were using San Francisco as a fleet academy, there were a whole lot of officers there, and eventually San Francisco became where Star Fleet command was headquartered.
One other thought though, the people on NX-01 were some of the most foremost experts in their fields, Archer recruited his Comm officer from Academia, Archer himself had been assisting his father with his work on the ship itself, a lot of these people were possibly recruited straight from being Frontline experts in their field or from Academia and college.