Perhaps it's all just good old fashioned human stupidity and refusal to be humane and lacking in humanity.
If you think of "humanity" as an adverb, a positive description of positive behaviour, rather than just a noun as in "all the people", it might be a key factor here.
EDIT: And when I say "old fashioned", perhaps there was an ancient interstellar ET empire based purely on power, with no room for emotional involvement or things like "pity" or "compassion". They've all gone now, but the echoes of who and what they were could be all around us.
Remnants of the dinosaur age are most certainly all around us, perhaps there are echoes of something older and far worse too.