ADDED: From Cracked's "7 Hilarious Monsters That Are Widely Feared in Japan," Nurarihyon:
I've often heard that the Japanese are a people who have a great regard for custom and formality, for the appearance of propriety at all costs. Doing things the proper way is important, and not being regarded as rude or obtrusive is something of value. And if that's true at all, then it kind of makes sense that they'd have a monster that is basically nothing more than a preternaturally shitty house guest.