Call for Skunk Theories re: Lost Civilizations

We so often hear of these possible/likely "Lost Civilizations" such as Who built the pyramids or Atlantis or whatnot...
What I want to know is HOW THE F DO WE "FORGET" AN ENTIRE CIVILIZATION?!?!?!?! For example there are the ancient (I think Sumarian?) tablets. The weathering on them is extensive. Where do these things sit for so long undisturbed but exposed to weather? These are supposedly revered items. Let's say 5 guys know where these things are being kept. You're telling me that all 5 guys died without relaying that information to anyone? Or that simply no one cared? Now take it to the level of forgetting an ENTIRE CIVILIZATION. Are you f'in kidding me? Something is wrong and I want some quality Skunk theories to mull over. Thanks folks.
-V

ps - or how did the Sphinx get buried? What, did no one have a frikin broom? There's this HUGE statue over there and not one person said, "Hey, there's like a foot of dust around the base of the Sphinx, how bout we go clean it off with a push broom?" Then, these people just F-off for thousands of years and let it get buried and packed down so that the dust becomes sandstone? COME ON!?!?!?!

Well it all depends on geography I suppose, a civilsation living in desert conditions could be engulfed in major sandstorms and those living near an ocean could be deluged, plus earth piles on top of itself over time.

The rest is truly a "how the hell" though.