Maybe they just don't feel the need to blast their emissions all over the sky like some sort of a caveman.

Our own technological signature is coming to form a very thin shell. Once we switch fully to fiber optics, lasers, and beamformers, there won't be any aliens learning English from listening in on our TV transmissions anymore. Radio broadcasting was cool, but also horribly wasteful.

It's probably incorrect to assume that more technologically advanced civilizations would be louder.