If the world were to unify into a single country then it would lead to endless conflict and strife. Who would be in charge? Americans would think they should be in charge. The Chinese would think they should be in charge. The Russians would think they should be in charge. Reaching agreement would be impossible. It is much better that different countries are able to rule themselves.
One size fits all doesn't work for clothing, it doesn't work for shoes, and it should never work for government. A single nation spanning the entire globe would be a bureaucratic mess, unable to give effective preference or priority to marginalized populations. And let's not even consider the nightmare of a military force that would be required to maintain order. It's just a bad idea all around.