Why would anybody be happy letting people with such fundamentally different belief systems make their rules?
For example, the US was instrumental in enacting the UN Single Convention on Narcotic Drugs, so a government that I don't get to vote for had a direct influence on the laws of my country (UK).
If, for example, Asian and Middle Eastern countries influenced the law of this hypothetical world government in proportion to their population, then the UK might be compelled to execute violators of the drug laws you refer to.
Of course this has the potential for conflict as well.
In the mean time we seem to be on a regressive path and for now further unification is over the horizon. The cause - at least, in my opinion - was that the unification attempts that were made were made in bad faith, for short term economic reasons rather than for long term stability reasons.