The reason human became a rarity is perhaps because in modern times people don't believe in the supernaturals as much as in ancient times, thus less demon contract/summoning, which leads to even less contact between the two races/worlds.
@SanitaryCockroach Since we're dealing with two different dimensions or worlds or whatever you wanna call them, they could be running with that "different passage of time" deal you see sometimes. A day on Earth could be much longer in the demon world. Alternatively it's just a joke and we shouldn't dwell on it that much because even two generations (from her parents to her for instance) is still "generations" in plural if we want to get a bit picky.
In the 1st chapter they say that demons eat human souls. How come in this chapter demons think that humans are a myth and that they may or may not exist?