Yeah, it's just impractical to try to lay claim to any significant volume of space, it's just too big. It makes much more sense to stick close to planets, moons, asteroids etc. where you have a much better chance to protect your assets. It's probably not even a good idea to try and defend the region around a local relay. I can imagine that not many races would risk a fire-fight that might damage or destroy a relay. That's probably how the Normandy was able to get around the terminus systems and even into Geth space without facing any serious opposition.