Humans occupy a relatively small space of the galaxy, at least when compared to the other races. Unity was never one of Humanity’s defining suits. Internal dissention for resources, territory and technology prevented Man from turning his attention to space. The invasion from the Kresh Empire changed this. For a brief period of time, up until the liberation of Earth with the aid of the Outcast Council, Humanity fought the invader as one. If anything, the war clearly showed that Man was not alone in the universe and there were countless worlds to be explored.
After the war, Earth once again crumbled into petty in-fighting but this time Man squabbled not over our own resources but for other planets. Eventually, the Earth Senate was created as a way to unify the planet and pool our resources for space exploration