I don't know if anyone is really arguing for state enforced equality. Just that in a capitalist system money naturally accumulates at the top and slowly regresses into a socialist like centrally planned economy as fewer and fewer people have meaningful wealth to allocate. A little inequality is good because there's a reward mechanism for allocating resources better but a lot of inequality locks up the economy. And the only thing to really do is tax it and recirculate it back to the bottom.