Because then all the countries would have to agree on a constitution and number of other laws. Given that cultures differ so much that sounds impossible to do. Also, that country would have to elect a common government where all the countries and peoples felt they were adequately represented. You can imagine the difficulty of that.
I don't believe a single world government will never come to exist. And that's a great thing. What if that world government started to oppress its citizens? Where would you go? What would you do if you had no voice and no exit, either?
Personally I'm of the opinion that nations of the world can never be united in one state they can only be more tolerant towards one another and keep their egos in check at every point in time. Unification of the world into a singular state can only occur when everybody in the world is under hypnosis or a genjutsu like the infinite tskuyomi. Well that's on a more lighter note.
Countries can't be united to form a coalition singular state not because there is ideology to sustain it but there is the factor of the human consideration.
First of is the language plurality. The number of languages that exist all over the world runs into thousands, whether its ancient, dead or forgotten it still counts as a language sonfar there are speakers of that language. This singular factor alone cannot allow for uniformity because the sovereignty of languages would be a major issue to be battled with. The singular state cannot adopt a. Particular language as the lingua franca because there would be dissenting voices from other language speakers. For instance you can adopt English as the official language when you have Chinese's with the most populations or Spaniards or latins etc. These other language speakers would contest that facts and it might end up in chaos.
The second factor is the factor of culture. There are divers cultures all over the world and every culture is individually unique. It would become quite impossible for you to say that citizens to abandon their traditional culture and take one a new culture that would be the tradition of the new state. This would only agitate the individuals and nations to quickly pull out because the sovereignty of their individual cultures would be at stake and eventually would be eroded.
Furthermore, it's no news that religion and belief systems play a major role in interstate relationships and human relationships and we all know that each religion is sovereign in itself. Every religion believes its the best and cannot be subjected to the rest. this factor would also make it impossible to bring the world together as a nation because religions orientations shapes human lives and once there are people with different orientations then there can't be peace and unity.
Also, the sovereignty of individual states and nations matters in the issue of creating a singular state. Creating a singular state simply negates the notion of national sovereignty as all nations would have to surrender their sovereignty to form a coalition state which is practically impossible as it is commonly said in international relations that there is no permanent friend only permanent interest and when certain decisions are not serving the permanent interest of a nation it would withdraw.
Conclusively, creating a practical singular state is a classical case of if wishes were horses even the poor would ride. We cannot achieve a singular state all over the world except we are all under hypnosis tended towards a particular order. But in reality the prediction of Marshal McLuhan on Global Village is the closest we can get to a singular state.
The world has become a global village and the spheres and dimensions of international relationship and communication had been vastly affected in a good way. With the globalization of information and the miniaturization of the media I can vividly day we are already existing under one nations state only that we retain our individual identity and sovereignty as individuals and as nations.
Because as long as we do not have a common enemy, a true union is impossible, we are too disparate and absolutely nothing is done to overcome the enormous differences that separated us; On the contrary, everything possible is done to confront the different groups and ethnic groups among themselves. The Power perfectly knows the maxim "Divide et impera".
Thinking about unifying the whole world to how we are today presents gigantic challenges:
Acceptance to the unfamiliar. Many people reject people and cultures that are not without affinity to their own. To overcome this problem we still have to expose ourselves more to different cultures so that they are not so strange to us. Technology helps a lot here.
Political system. How would this world be regulated? Would there be a "king of the world" like in Dragon Ball? What gives him or her the right to be at the top? Maybe you think of regional committees, what guarantees that everyone respects ideals and is not corrupt to use the power they have for the benefit of their particular environment?
Monetary system. What currency would be used for the exchange of goods? Also, what governs your valuation? Since the world is so big, giving a coin a value would be too complex. It would be necessary to develop something alternative, such as bitcoin, a currency based on nothing but supply and demand.
Religion. This issue I see mainly as a great impediment to getting the countries of the world to join. Religion, I think, is the burden that shows how primitive our society is. Sing and pay homage to idols ... God grant me.
Pareto Law. The Pareto Law tells us that 80% of the population would be in 20% of the territory. This involves several challenges and challenges of housing, justice, social development and others. Wealth would follow this principle in one way or another. So several more things.
A society with the universal perceptions that we have today is impossible that is not conflictive. We are vastly educated believing that there are good and bad in history, we admire countries that speak of freedom but punish and are too intolerant inside if a neighbor leaves his grass to grow more.
I look like this, if there can not be unity within a single country with the mental evolution of today, there is still a long way to go before we think we are ready for something global.