[throwaway294566]

Because "rich", in many definitions, is a moving goalpost. If you define rich and poor as related to some global average, inevitable differences (we are not a Borg collective) will always produce some rich and poor people.

If you, however, define "rich" and "poor" as related to a fixed criterion based on the necessities of life, like "will/won't suffer from starvation during their >60 year average lifespan" or "child mortality <2%": then the world is steadily improving and humanity has never been richer.