Its because the humans living in the human village are supposed to be off-limits for Yokai. Its why if you want to stop being human, you need to eschew the protection of the human village and the Hakurei Shrine Maiden, otherwise you risk being exterminated.
If the human villagers aren't getting eaten and could "safely" watch the yokai eat outsiders only, then those villagers assume they are safe and won't be scared of Yokai anymore.
But if you have a human villager get snacked on every so often, that fear will never go away. So having the occasional lawbreaker eat someone in the village, or grabbing a villager as they forage outside of the village walls ultimately keeps the balance of Gensokyo going. The yokai need fear after all, and there's only so much fear you can get from chasing down outsiders. The villagers when properly managed provide a stable supply.