It could very well be that the only civilizations that still exist are the ones that are good at hiding. All the others (even if they are the big majority) might have been destroyed after giving a signal of life/existence.
I'm guessing that it's not unlike why we don't leave food out when camping in bear country. If the bear realized what we were doing (hiding food from it), then it would surely come to the bear conclusion that our behavior was unreasonable, but we know better.
Because they might attract the attention of greater beings.