What if the reason aliens are uniform across their world is because every species has a deep, intrinsic connection with their home planet. So colonising/terraforming a new planet is considered a form of disownment, as it breaks the connection with your home world. Humans connection is much weaker than most other species, but it allows us to still see humans across the galaxy as HUMANS rather than traitors, or a different "breed" of humanity. -- Sarah
[AN: Obviously, this can't happen in my pet universe. Slight pause whilst I build a new one...]
The greater expanse of civilised space is full to the brim with statistical outliers. Those who chose to forsake their home planet, their mother orb, to make a new place somewhere far from where they began. Except for the Humans.
They had no boundary to the concept of 'home'. They could take up temporary residence in cookie-cutter living quarters and, within an astonishingly short amount of time, they would be calling it 'home'. They could call anywhere they hung their head-garments home. And they had an unending curiosity surrounding the concept of 'next'.
They were fast, and tough, and endlessly adaptable. Each new world was merely a place full of stuff they could use. And they used resources relentlessly. Always looking forward. Rarely looking around themselves at what they were doing now. They were everywhere and they were baffling.