Because humans evolved in a tribal setting with at most ~200 people per tribe (see Dunbars number), and as social animals have a very keen instict for in- and outgroups? Any group larger than that number (and often much smaller) will start to fracture into sub-groups that then compete with each other for status and resources. You can see this effect even in groups as small as 200-person startups, where there can be vigorous "us vs them" dynamics between the dev and marketing teams or even between dev and ops.