Maybe if the monsters were self perpetuating but it seems most of these things are more like a disease than a separate species. Vampires, werewolves, shifters, wendigos, etc are reproducing only because of a human victim. Fairies don't require humans and they have their realm - BTW did Dean & Sam figure out that it must have been a banquet table and not a probbing table Dean was being lead to? I guess fairies can't live on cream alone. As to knowing where Heaven & Hell are - it could be instinctual for both Angels & Demons - they are naturally drawn to their place.