Because with the new F1 A8, you get 5670 level graphics (I believe) with athlon x4 performance. A comparable CPU is $80, and the GPU is $80. That makes it more expensive to go that route.
Also, it's much lower wattage than running an external GPU, meaning it's much better for HTPC's. It also gives off much less heat and is more efficient and very low profile.
Basically, it's an HTPC/casual gamerbox product. As shown, it beats the pants off the i3-2100.