They earn 4x more because the world is not truly flat, and there are artificial/natural barriers which hamper competition such as; movement of humans, scarcity

They earn 4x more because the world is not truly flat, and there are artificial/natural barriers which hamper competition such as; movement of humans, scarcity of taxi medallions along with fixed pricing, input costs of a taxi ride (cost of living, gasoline + subsidies/taxes), labor laws etc.