Because of the nature of natural cycles. Mankind can, locally, prevent tides from coming further inland with dikes, or harness them with wave-generators, or even enhance them if they want - but they cannot change the fact that the moon and the sun and the Earth's rotation combine to create tides. This is, comparatively, a 'shallow natural cycle'; it has power, but it happens in a 12-hour occurance, with variations throughout a 28-day lunar cycle.
Move to something deeper: the seasons. Mankind can (has!) altered to an extent the seasons - we've increased the general heat level of the planet, resulting in generally hotter summers and generally warmer winters. But mankind cannot even come close to altering the fact of them; that is generated by a 365.24-day-long cycle, the Earth orbiting the sun.
Move further on - imagine a cycle that takes over 10,000 years to complete. Humanity can, in certain locations, and with an immense amount of effort, heighten or dampen the effect of the mana cycle. Shedim 'took advantage of' one of those incidents - a deep astral rift and the mana spike of Halley's Comet (a 76-year cycle that we COULD affect, due to its being a comparatively small item to affect.) If the two hadn't been combined, the shedim wouldn't've arrived for at least decades, and probably centuries. The other nasties are working on a similar restriction - they can only be summoned at certain strengths/flavors of mana (mana spikes, such as that of the Great Ghost Dance, or the Corrupted Locus in Aztlan), and being able to naturally cross is something that requires even higher levels. Those levels are not due to be reached for an estimated two thousand or more years, because of the very nature of that massive cycle.
If you want to use them, by all means go ahead; it would radically change the nature of your game, because they ARE arriving centuries before they should, and metahumanity is Simply Not Ready For Dealing With Them. The mystic technology (so to speak) of the kaer is yet to be rediscovered; it is clearly something more than 'just a big ward'. IMO, the arrival of the Horrors would be something to make insect spirits, CFD, toxics, Fear the Dark, the HMHVV and VITAS plagues, and both the Crashes all rolled together seem like a happy-go-lucky Christmas party. So use them if you want - but remember/realize that you're dropping the worst of the worst on a world that isn't ready for it, and simply can't be ready for it for another few decades, which means the Horrors ... win.