[kolinko]

Well, another option is that it takes - say - 200 years to get from the stage of "we're producing enough radio output to be visible" to "we developed cloaking" or "we switch to other tech that is intentionally or not - not visible".

If that's true, only a very small fraction of planets with intelligent life would land in the window where we can observe them.

Another option is that you might have many civilisations that are just as intelligent, and just as developed, but not in the same fashion - imagine 99% of intelligent species out there are more like dolphins than like humans.

Or their atmosphere is just a bit more hazy than ours, so they are not aware of the night sky filled with sky, so with less curiosity to imagine the world outside of theirs.