The sky teems with life. Most isn’t sapient. By the time colonization of the galaxy concluded in CE 27000, the bakers dozen of known sapient species known in the days of the proto-empire had expanded to well over a thousand, with non-sapient species numbering in the trillions.
Humans were startled to find that that not one of those thousand-odd sapient species had colonized even a single other planet. Even, for example, a one in their home solar system, mere light-minutes away. Worse, while a few had primitive space stations in low orbit, the majority hadn’t even that.
This level of space inferiority was all the more surprising for the cultural and technological marvels the surfaces of the home planets themselves boasted: undreamed-of advancements in medical and physical and mathematical sciences! So it obviously wasn’t a question of intelligence (which was, besides, approximately equal to that of humans). Therefore: “Why?”, was the question the explorers asked their (usually congenial) alien hosts.
There was no ready answer, but gradually sociologists cobbled together a theory: laziness.
As anthropocentric as it might sound, the theory made a good deal of sense. Getting off your home planet is technologically difficult–a challenge made harder still when you have to inspire a population that, by definition, has never left home, to the requisite elevation of perspective. And so it was that none of the thousand-some species, save humans alone, had ever made any progress beyond those first few, symbolic rocket flights. Imagine the catastrophe if humans had been counted among them.