That argument relies on the utilitarian assumption that maximizing the global well-being is the goal. For most people, that is not the case, and understandably

That argument relies on the utilitarian assumption that maximizing the global well-being is the goal. For most people, that is not the case, and understandably so. We care about our neighbors, but we don't care about alien lifeforms on other planets, because we have no contact with them and no way to help them. Same can be applied to poor people living on the other side of the world. Where exactly do we draw the line? The simple answer is that helping is inherently selfish - we help people because we expect to be helped when we are in need (for those who disagree - imagine being in a community where everyone asks for help, but nobody gives it to you when you need it). So the obvious answer is we need to help people in our proximity. Of course, like many other human affairs, there's no simple algorithm to decide what do to in a general situation. It all comes down to an individual's will.