Because it’s a lot easier to lose a single planet to threat forces than it is to lose many? Because one secret nova bomb could end a war before a war is ever declared? If your entire 150 memberworld Federation, with thousands of colonized planets, can be taken out because you put all your eggs on Earth (because, hey, kids, we’re the tops!) then you deserve annihilation, and, let’s be real, Star Trek is a stupid show. Hell, that’s the point behind the decentralized nature of the internet, among others.