A strong centralized government would probably only bring about more war in the world. There is no way that all of the people in the world will ever agree on important issues so separate countries are best. Strong centralized governments only make people angry and slavelike. Freedom is what makes life worth living.
There are far too many cultures and societal norms throughout the world to have one code of law that applied to all. Even if the world recognizes that all humans share certain rights, enforcing said rights in a population of 7 billion people is essentially impossible. Humans can't enforce bans on slavery, how can we try for more?