The reason why youkai happily chomp on outsiders, and not on villagers, is that they are responsible for producing the baseline fear/belief for Gensokyo. We can assume the purpose of the Hakurei God in this scenario is effectively protect said fear farm, as if youkai were to eat the village humans, said farm would rise up in arms and would battle back youkai. It's also why spell card rules are in place - it ensures the superiority of youkai and "exterminators" (meaning: balancers who keep youkai in check) whilst also protecting the belief supply. Outsiders are intended for those that need more than acknowledgement with fear - effectively micronutrients and goodie bags to be simply put.
The most egregious example that stands out to me is the kappa. They require salt to survive, and they get it from humans. And the thing is, they don't have to. By drinking blood and eating shikrodama (ass balls), they maintain their salt intake.
The worst part is that they don't need to do so - in WaHH, a kappa mentioned that having access to salt water would liberate them from consumption of human flesh and blood.
It shocks me on how deaths can be prevented in cases as these. Yukari could easily gap in salt by the tons, plus it would help youkai-human relationships as the prior needs to consume less of the latter. Of course, she's a immature bitch like the rest of the power players in Gensokyo (which disturbingly follow a trend of: stupid, immature, sociopathic, or all three) as evidenced by "muh second lunar war" which would have resulted in Gensokyo being wiped off the face of the Earth if the Moonies reacted (and probably a good thing).
Back to balance, the purpose of youkai exterminators are to balance Gensokyo as the village believe feed belief to youkai stay safe as if they were also chomped on they would run out of food. That's why Outsiders are brought in, to feed youkai like Remilia and Rumia. The youkai that consume humans are fed, while villagers still produce fear in peace. That also goes hand in hand with weakening the population and effectively making the Hakurei Shrine maiden mandatory to protect humans and enforce the status quo, while preventing them from gaining true power through keeping Reimu in the dark, and keeping the Hakurei god down but not dead.
The youkai who break this status quo are taken down by exterminators, as they risk destroying the balance that keeps all the youkai fed.
Youkai are parasitic gods
This balance only serves youkai
The Village is a farm
Hakurei is a traitor
Man must be kept in the dark
The truth will erase the faith
The lies must be enforced