>>724851073 (OP)
we as a species are extremely territorial and will engage in violence to protect our land, our property, our way of life. Our various conflicts as a result hampers our ability to become a space-faring species
The common theme here is that these alien empires are already space faring, often long before humanity was. Assuming they had a similar technological and societal path as us, the natural assumption would be that this is an alien empire that came out victorious over all it's enemies, and now has a mass of wartime technology that has been used, counteracted, improved and perfected well and truly beyond what humanity has achieved. And so we become the bottom of the galaxy in most fiction.
tl;dr fiction writers fantasize about being stomped on by a military boot.