A worldwide government, at this juncture, would require the subjugation of various cultures, laws, morals, religions, among other things. This is not happening in a bubble. You are talking about putting mortal enemies under one Government, and forcing one side to share their resources with the other. Really, what you are asking is that everyone give everything up for the greater good. You are not only advocating a world Government, but you are pretty much proposing Communism.
What you will have is the reason that Communism, in pure form, does not last. In theory it's a grand thing. No one starves, no one's out of work, everyone gets the same thing. In reality, you are removing competition, and you are delegating the "everything" to a select group of people to keep watch. All our food goes to, for instance, a "Food Czar" I guess. What happens is that, when someone wants more food, or better quality, they will sip the "Food Czar" a bit of something he wants. It could be a service, a trinket, or whatever else. Suddenly, things aren't so equal. Suddenly, we are bartering for things and not everyone is on equal footing anymore. This reintroduces the possibility that some people will have little or nothing in the long run. Everyone loses everything, except for the few who control who gets what. This is why "Communism" we see today tends to reveal a curious disparity between the leadership (and usually the military) and the people living in "equality".
With a single Government, there is a complete lack of choice in the matter. I like the hodgepodge and variety our world presents at the moment. There are countries for people who enjoy theocracies. There are nations with nationalized healthcare and just about everything paid for with taxed. There are others where this isn't the case. There are cold areas, hot areas, arid, humid... it's endless. When this single Government decides to do something you hate, what is your option? Go to the moon? Lobby, plead, and petition?
With a single Government, if we are going to make it a Democracy instead... well you would need to think of the logistics there, but let's assume for a moment our fearless leader will be elected. The 7 billion people on the planet are going to vote. How many of those are Chinese and Indian? What sort of representation do you believe is going to be achieved when those two nations gobble up the bulk of the votes? Keep in mind this is not some ideal China or India... this is the nations as they are now. If we are not doing things by population, then perhaps by land mass! Wait... hmm... Greenlanders seem to get an astounding percentage of the vote. What about Canadians? So much land unused and unlived-in. No, that wouldn't be fair either. How about we just forego borders? But there were borders before, and there are still cultures, no matter how much of a gooey global melting pot you want to pretend it will become.
There will still be people living in areas prone to disaster, or will our Government get us moved away from there? There will still be people living in areas where transportation is incredibly difficult or impossible, or will our Government focus efforts to move those people or build roads/bridges? It seems like there would be an awful lot taken from people that worked very hard to earn it, to do what the leadership deems "important". What is the incentive to work when what you do doesn't benefit anyone you come into contact with?
Ah and here you will say "but don't you want to help your fellow man?!?" And that is the ultimate straw man argument. You said you do want to share. Who, right now, is stopping you? You can have homeless people at your table. You can rent out rooms to the needy. You can volunteer. You can adopt a child or twenty or whatever. You can visit the elderly. You can contribute money to good causes. The problem I have is the audacity of someone proposing something like this, to tell me what's worthy, and to deign to tell me what should be done with my resources. I won't even say money, but resources; time is finite and precious. The people who contribute most to the "have not"s in this world are people who have earned their money.
Forcing people to be all perfectly equal never works, anyhow. There's always something. The logical next step down this slippery, mud-caked slope is to say that some people will feel inferior to others because some are prettier. Free plastic surgery for all! Some are taller, or shorter, or get arthritis, or have diabetes, or whatever else... in short: life is not fair to us. There are ups and downs and it's up to us to work our way out of it and, if we so desire, to help others as we can... not to bring every human being down because it's too hard to do anything on our own.
