I think there are a few reasons people gravitate toward the difficult questions posted on this board (or located elsewhere.)
On one level, I suspect people like the challenge and the attendant ego boost when they solve a difficult question. I know I enjoy it.
On another level, if you are seriously targeting a score above 700, you will need to practice those challenging questions.
As for why some prep literature has seemingly fewer of those hard questions relative to to what you find on this message board, I think you have to consider demographics. Test prep books and programs have to target as many people as possible who will take the GMAT. GMAT takers are not all from Lake Wobegon (where children are all above average.) Those who may expect to score at 700 or above are going to be a minority of the population. Test prep publishers would alienate the majority of potential customers by focusing on the most challenging questions possible on the GMAT.
This site is really a community of GMAT enthusiasts, a disproportionate number of which will likely be targeting scores above 700. As such, you would expect to see more of a focus on very challenging questions.