It's interesting that effective altruism tends to focus on the world's poorest people (to help them) and the world's most privileged people (to help them to help others). Why not more action in the middle?
At the bottom end you get a big gain due to the logarithmically declining value of extra income. The world's poorest people benefit much much more from extra money/healthcare/etc.
At the top end you get a big gain due to the power law distribution of income, audience size, influence, useful output, and so on. The 7 million people in the top 0.1% have much much more capability to improve the world than the 7 million people in the very middle of those distributions.
Unless a similar underlying phenomenon is discovered that gives a powerful reason to focus on the middle, I expect we'll continue to see this U-shaped allocation of focus.