The only reason why this is not happening is that there is more than one such "federation" currently and no one is unquestionably stronger. And at no point in history had a stronger faction submitted to the will of a weaker faction just to avoid hostilities. In a sense war is just a reality check when it is unclear who is really stronger. While studying history, it struck me that many countries (current unit of sovereignty) honor some person who was a "great unifier" - created a cohesive whole and put an end to internal conflicts. As a rule, this person was a war-mongering psychopath.

As you notice, at the end of the 20th century we almost had this kind of "federation" when the US and its allies were the only center of power in the world. The US arrived at that point by winning an economic and cultural war. But its coalition failed to entrench itself, grew weaker and more bloated and now we are facing these dangerous rifts between different factions of humanity.

Note: in the current emotional climate this may sound like war apologism. It is not - war is unspeakably horrible for everyone involved. But this is what an impartial alien looking from an orbit would deduce about the humanity.