so governments can create war
Because the people on this planet are retarded
It's the modern incarnation of tribalism.
When humans began farming (staying in one area as opposed to being 'hunter-gatherers' and having to be on the move), they stayed in one place and needed to protect their tribe, settlements and farms and live together. That evolved into bigger settlements to town and cities along with a ruling class. From settlements to cities to countries; people have always been separated between us and them.
A question that's answered way back in early human development... Because we gathered, cooperated, soon there's too many people, we needed a system, got the system, oh look there's another group who could be potential danger, we needed boundaries to mark our territory so we could survive, and now today it's just a thing that we do because there's so many people on this planet.
Because some humans are wolves.