A single inescapable fact is that humanity united with infinitely greater purpose in pursuit of war than they ever did in pursuit of peace. -- Anon Guest
Humans are not good at sharing, they say. Their origin planet was divided into thousands of groups identifying themselves with borders, religion, and culture to define battling clusters. When all of those were in common, lines divided amongst economic, gender identity, skin tone, and creedic boundaries.
All it took was one contact with visiting alien life for all of those to no longer matter. Peace on Earth only came about with the identification of someone Distinctly Other. Only then did the nation-identities of Earth merge into one, solid unit. Allied against the outsider.
One can only imagine their disappointment when they discovered that the greater portions of the Galactic Alliance had next to zero interest in invading, conquering, or purchasing a Deathworld that was close to ruin due to the actions of its original cogniscent inhabitants. Further, several colonial planets wished to have serious discussions in regards to Terra's stated 'garbage dump' policy of treating wormholes as a solution to 'problem groups'.