The underlying nation is called the world wide web. Whoever feels otherwise is ignorant. Actually 17% of Indian is leading example of this a working model of why this is 100% possible. India was more than 600 kingdoms and has more than 4000 languages and since its independence for the last 70 plus years it is striding forward peacefully. This was the same model tried in European Union. When the individual's ego of leaders go beyond the common goal of people needs it breaks down. When the idea is to bring all together you bring thoughts to do it but if you choose your idea to break it and keep it separately the same conscious mind will help you do it. So it is you who will able to make the change. The change starts with you. So please start believing it is possible and it will become possible.
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.