The thing is, in STD Earth has now been attacked TWICE and still no defense system was anywhere to be seen. In Best of Both Worlds, we got a glimpse of primitive solar system defense, and well the whole Earth defense fleet gathered at Wolf 359. The fleet got complete destroyed, that's why there were no ships left near Earth. The second time Earth got attacked, there was the First Contact big ass space battle with countless of ships. And in Voyager Endgame, there was again a whole fleet waiting.