For instance, if our best Earth tech was pointed at Earth from a distant star, how far away would we be able to detect our own traces of life? We know that life is "a little rare" at least, but there is a huge range of rarity where life is everywhere across the vast, vast universe but sparse across the dinky number of light years we could see each other. Maybe aliens have tech that is 10,000x better than ours but the scale of distance is just too big of an obstacle.
There is an assumption that alien races must have figured out things well beyond our understanding and be doing things like harnessing stars in some dramatic way, but maybe that just isn't a reasonable thing for life forms to do? Or if they do have that ability, they may operate on a much longer timeline, simply nudging things along in a way that escapes notice.