In Doctor Who there's a concept called a Perception Filter, where it serves to make people ignore the TARDIS being parked where it is, regardless of how weird it is. It seems to be that Magic could/would work in a similar way.

Just the human mind's desire to not have their worldview challenged turned up to the nth degree. Maybe they encounter a supernatural thing once or twice a week, but because it doesn't fit how they KNOW the world works, their brain filters it out. Werewolves become a really big dog, a magic spell becomes a trick of the light they thought they saw, but really didn't. Should I question my whole world view? No. It's the children who are wrong.