honest question: what's BAD about having a single government?
if i had to pick one thing, i would say its too easy for a single goverment to be corrupted, and a corrupt goverment of that size would have absolute power todo whatever they wanted.
its better to have hundreds of different flavors and levels of corruption with the occasional good goverment(i'm sure theres 1 or 2 countries out there in the world), that way no1 can go and fuck the entire world over(even though people are hard at work trying)
a runner up reason would juse be that many cultures are so fundamentally different(beliefs/ideals/etc/etc) that their respective goverments function as such. theres no chance of one mass of people understanding another mass of people across the ocean, a flaw of modern humanity and society.