If the Ice Age Ended circa 10000 b. c. to Submerge Megalithic Ruins Then Why No More Built for 8,000 Years?

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Biblephobes say the Ice Age ended circa 10000 b. c. when the sea level rose to submerge megalithic structures found for instance off the Ryukyu Islands of Japan, in the Black Sea off Anapa, Russia, many sites off Greece, and Kushasthali in the Gulf of Kutch off India, so if they were in fact submerged that long ago, then why were there not megalithic structures built during the ensuing 8,000 years up to the time that such as Baalbek and the Great Pyramid were endeavored?