The Secret of Magic
There is an age-old myth that the supernatural is hidden from the world of man by powerful organizations which uphold and enforce compacts of secrecy. This could not be further from the truth. Sure, those of magical means don't typically air their dirty laundry, trade secrets and all, but there is no regulatory body on the matter. If so, why isn't magic more well-known by the 'civilized' world?
Magic hides itself. It has done so since at least the 16th century, not long after Columbus sailed the ocean blue. It only got better at doing so around the start of the industrial revolution. Evidence of Magic from before is bountiful, but labeled as myth and thrown into the bin with other irrational beliefs. This only further helped Magic in its aims. The leading theory is that while humans have always abused the World and each other, they only got better and better at it over time. Thus, to protect itself and those that it nurtures, it had to disappear into obscurity.
Magic isn't intelligent. It may seem so, but it is more akin to a hurricane than a thinking being; it reacts to the forces of the World in ways that seem like it is alive in the typical sense, though it is not. It may disappear a body half-drained by a vampire into the local shrubbery. It may push an active war zone between chaos-inclined cats and pigeon familiars into a forgotten alleyway. The river may swallow the lost bones of an unknown saint until the right person passes by. Even a headless cyclist may go unnoticed by those without a discerning eye, trapped in their own busy lives. Even a summoned tentacle horror downtown can be explained away as an unexpected earthquake.
There are, of course, people in the know. Most of the time, their interests and Magic's own align. When something is simply too big and too out in the open for Magic to hide on its own, the denizens of the magic world will join together. Mages and monsters who would normally kill the other on sight will willingly cooperate when one of their own attempts to overly influence the human world.
The human world still reigns supreme, no matter what fancy fireball or resurrection trick one may know. Their numbers are too great and their methods too cruel. The human world can kill anything it cannot explain. This is its greatest gift, and it will do so. The only exceptions, of course, are the end times.
And nobody wants to see those.