Because otherwise golems won't spawn in the spawn area. Golems spwan area are in the exact center of the “village” as recognized in the Minecraft code and unless you're very in the knows about how Minecraft calculates the center of the village, you won't be able to know where it is, and so the killing area of the golems won't work.
By moving the farm far away, in a set configuration of the farm, the golems will always spawn in the kill area.
Golems will spawn withing 7 blocks of the villager. stats as I understand them are 6 above, 3 below, 7 N,S,E,W. My spawn area is above the villagers and is a 4x4 area with a lava pool for destroying them, all other blacks have been spawn proofed. It is a very small farm but good enough for just me.