Yeah, because living your entire existence in hiding, and instantly annihilating anyone else you come across just in case, is totally a worthwhile way to exist. Do you win if your civ makes it as far as the iron star era?
For all we know, the universe is jam packed full of Predator-like aliens who are the dominant species on their planet specifically because their society is built around dominating everything they can.
Your point doesn’t really hold up if the effort to destroy a civ is basically zero, while the cost to defend is approximately unachievable.