It's a generally recognised principle in economics that we want competition to take place, and for consumers to have a choice - for a plethora of reasons too large to list here. The most important IMO is a political one though: we don't want monopolies for the same reason we don't want monarchies and dictatorships. Being the sole provider of any very important product puts an obscene amount of power in the hand of a single corporation (or a handful of them). We never want that much power to be so concentrated in so few individuals, because it's fundamentally injust, and always leads to catastrophe. Also consider that if that power is a corporation or a "private" individual, they're not accountable to anyone but their shareholders (who, in turn never want the corp to be anything other than a money making machine). Even a dictator is more accountable to his subjects than a multinational corporation is to anyone. So there you have it, that's why we need alternatives.