Having more external components can add cost in many ways - here's some that spring to mind:

Extra design effort for the schematic;

Extra design effort for the PCB;

Extra cost to produce the PCB;

Extra cost to load & test the PCB;

Extra cost to maintain the BoM;

Extra purchasing, inventory, etc, costs

Anyone care to add more...?


Of course, these may or may not be matched by increased costs in an "all-in-one" device - but it all goes to show, again, that focussing only on the chip price is likely to be misleading.