1a) WHY CAN'T THE MILITARY DEAL WITH MONSTERS?
The militia and other military forces are low-level in my campaigns (which are in 3.5e). The setting doesn't "flex" to fit the levels of the PC's, it just is what it is.
Yes, but where you have 4 PCs a large city might send 400 soldiers. Will there be casualties? Sure but they can also get the job done without relying on Slayers.
And considering the wandering nature of Slayers it might be preferable to send your forces instead of hoping that a Slayer strong enough to deal with the threat shows up by chance when you need him.
The overall point is, if civilization is really so threatened as described in the first post it is unreasonable to have your best defenders wander around aimlessly with no sure way of contacting them.