Reply #2: Maybe because they were made of wood

[Papa]

and that the knowledge was "secret" even in those days. Knowledge was power. It's a good question however.

or, on the other hand, just because we havent found it does not mean that it didnt exist. Have you heard of the Antikythera Device? http://www.world-mysteries.com/sar_4.htm

Only one (so far) of it's kind has been found and according to what we think we know about ancient history it should not exist. It's over 2,000 years old and is a mechanical device with gears that tracks celestial objects (maybe used for navigation).