I'd have to dig around to find the source - but honestly, its because we dont have good drawings, or even partially complete understanding on how to build the pieces. The Saturn V was not mass produced, and was built on something less then an assembly line - it was semi-bespoke basically.
On a related note, ars technica has an absolutely fantastic article on reverse engineering and re-building the Saturn V.
http://arstechnica.com/science/2013/04/how-nasa-brought-the-...
Saturn V remains the tallest, heaviest, and most powerful (highest total impulse) rocket ever brought to operational status, and holds records for the heaviest payload launched and largest payload capacity to low Earth orbit (LEO) of 140,000 kg (310,000 lb).
To date, the Saturn V remains the only launch vehicle to launch missions to carry humans beyond low Earth orbit. A total of 15 flight-capable vehicles were built, but only 13 were flown. An additional three vehicles were built for ground testing purposes. [0]
So why aren't we just building more of these damn things and iterating on the (proven!) design? While we're at it let's un-mothball the SR-71 and those wicked Buick start carts! [1]
[0]: https://en.wikipedia.org/wiki/Saturn_V [1]: http://www.sr-71.org/blackbird/startercart.php