Because they provide services that content mills don’t provide.
True, there are excellent low-paid writers in every country. I understand that they can charge less because their expenses and cost of living are lower than in other areas. However, I always wonder, if you know that your neighbor can make , for example, twice as much as you for doing the same work why would you charge less than your neighbor? Wouldn’t your standard of living improve even more if you upped your prices to what the competitive market bears?