system
Given the following post, respond as if you're commenting on Reddit:
human
[Buffy the Vampire Slayer] Why keep the vampires and other bad things secret?

Doesn't that only benefit them? If everyone knew what to watch out for, including trained police and soldiers, wouldn't everyone be safer? Also, Buffy wouldn't have waste time going to school and dealing with normal person stuff.
gpt
The main reason is to avoid an arms race.

Demons find it useful to have an unsuspecting human population to draw on for food and labour, and by keeping their presence hidden, they don't have to spend resources on actively subjugating humans or protecting their own interests. Very few demons have any actual interest in actively controlling humanity and their needs from us are fairly limited overall, so it works out for them to try and keep themselves (mostly) hidden.

The Slayers and Watchers focus on taking down the big "want to destroy/enslave humanity" threats, and otherwise just cull the heard and make sure the demon population doesn't get too out of control - especially when a Hellmouth starts getting active. Holding up the Masquerade benefits them by keeping the scope of the conflict small. So long as it stays in the demons interests to stay on the down-low, the overall threat to humanity is minimal.

Now what happens when you try to break the masquerade, and make demons and the like public knowledge? Well then all your miscellaneous vampires and small-time baddies start joining up with Cult Leaders and Big Bads like The Master or Angelus - strength in numbers, no one wants to find themselves alone in front of a lynch mob. Now instead of a small group of true believers and subservient minions, they have an army. And an army of vampires or other demons is really bad news. Police or military weapons and tactics are not at all equipped to handle opponents that can only be killed by a wooden stake through the heart (high explosives to remove the head nonwithstanding).

Another thing to consider is that while modern armed forces may have a chance to rally against a demon army, this is a very recent event in human history. Two hundred years ago the human population had only just passed one billion people, we were fighting with muskets and flintlock pistols, and didn't have anything like the communication tools or strategic command ability we do today. Open warfare in the 19th century would likely have been a curb stomp in the demons favour, and coming from that mindset, the organizations today don't want to take the risk.

Also, when the government and military *did* get involved, they started meddling in forces they didn't understand and generally screwed the pooch on things. Though that group may not have been representative of government/military efforts as a whole at preparing for the supernatural.