D5(+++) Puzzles

What is the appeal in making a puzzle cache so complex that no one can solve it? I can't imagine what could possibly make this fun for a CO. I've seen some caches that have been published more than 3 years with no one solving it or finding it. Some caches are found by other means, and others that have been published for almost 10 years with just a handful of solves (and a handful of finds from others who got the co-ords handed to them).
I'm curious - do you like to make puzzles that can only be solved by a select few? Have you come across any caches that have sat for years unsolved?