[roywiggins]

Well, yeah. It's really hard to do, so maybe nobody does it, or it's just impossible. But with several billions of years of galactic history it has to be pretty hard if civilizations are common.

Really it's just one example of a technosignature that is longer-lasting than radio waves, that extends the amount of detectable overlap we might expect to have with an older civilization, and the lack of observation either says that sort of technology is either not feasible or there aren't many civilizations in the galactic past or present.