Why do doctors count monocytes and not macrophages in a peripheral blood smear?

They aren't there: Macrophages reside in tissue outside of the blood vessels. Monocytes exist the blood and then transform to become macrophages (or macrophages repopulate themselves, depending on the location). Thus, only monocytes can be found in the peripheral blood. There are several subtypes of monocytes that we will begin monitoring in the near future to assess for certain diseases and treatment effects.