Another factor that as far I remember was present in the novel was technological acceleration, by the time you detected the first tries of "turning on the lights" of an emerging civilization many light years away, that civilization is not an emerging one anymore, and even more by the time you can get there, and they may eventually be able to do something to harm or destroy your own civilization, so it is not something that should be left unchecked.

And technological acceleration is a constant in that universe, the attackers were just a bit ahead of us in technological advancement, lets say a few hundred years, not the millions or billions of years ahead of the very bad ones.