Ward,
Difficult holes that appear further into the round tend to be mentally assimilated better than opening holes where the fear factor and unknown is more pronounced.
Likewise, when you've got a great medal round going, finishing the round, or needing a par on a difficult par 4 adds additional elements of fear and the unknown.
ie, if you needed to make par on # 18 at WFW or Riviera to shoot your career round, those holes tend to be far more difficult than if the same holes were in the mid-section of the course.
It's that special, excrutiating pressure that the last hole places on the golfer that can make the 18th hole so formidable, moreso than it actually is..... physically.