Earth almost always seems to have a very light Starfleet presence in terms of big ship count (not counting small shuttles or civilian passenger liners). In TOS and TNG, whenever the Enterprise visits Earth, it is usually the only ship seen there. In the TOS movies you see a couple other ships in Spacedock and that's it. In Star Trek Generations, the Enterprise B launches from Earth and then begins heading towards Pluto when they receive a distress call and they are the only ship in the area to respond. In the finale of the first season of TNG, Picard becomes aware of a conspiracy within Starfleet Command, and the Enterprise along with three other ships begin making their way to Earth to confront Starfleet leadership, and apparently a gathering of this size is unusual. And so on. So the Enterprise and the Vengeance being the only big Starfleet ships around Earth can make sense (though if the battle lasted more than 20 or 30 minutes then other Starfleet vessels from nearby star systems probably should have shown up), though as you said they should be receiving hails inquiring as to what is going on.