I've yet to be convinced that "Why can't we hear the aliens" is at all a sensible question. Beyond our nearest star systems even detonating nuclear warheads in morse code in high Earth orbit will get drowned out by cosmic noise pretty quickly. Space is just too damn big and empty.

I'm more interested in the question "what would the aliens need to do, and how far can they be, so we can detect their signal at all with our current instruments". Any good sources on this? I found this article, which is interesting, but very pointedly treats the observable radius of a signal emitter as a free parameter, essentially ignoring it as a limitation.