It is really unfortunate that most Suspense/Horror films confer this very contradictory dynamic upon themselves. What truly drives the adrenaline response when watching these movies is that fear of the unknown, a vicarious feeling of helplessness, in so far as we cannot logically analyze what we would do in a similar situation if we are not aware of our 'enemy'. However, the monster, villain, creature, and so forth, must always be revealed in the interest in linear plot development; unfortunately many, or moreover most, major studios are not willing to gamble on a Blair Witch, or Alien-esque movie in which true suspense is created through the absence of, as opposed to the encounter with, the enemy.
As you stated, most movies of this Sub-Genre often lose my interest, and consequently respect, within the first hour, as this is often when we first meet the antagonist. On a side note, my favourite horror movie of all time was Event Horizon