I agree. I even suspect that "some" of those civilizations were sufficiently old to have been around during the last Ice Age, and their sites are now well-submerged. I find the notion that there was at least one such in the East Indies/Malaysia area that is theorized to have been the source from which civilization spread from into Egypt and Mesopotamia, was well as lesser known examples further east--the diaspora having been initiated by the flooding of the home of the "core civilization" at the end of the last Ice Age. I think there are one or two articles here at FR that allude to this.
Why would those sites be submerged? The opposite is true. they would be on high and dry land now, because as evidenced by the viking mooring rocks, the massive glacial ice sheet pressed the continent down. Much of our coastlines as we now know them were under water way back then. As the ice recedes, the continent rizes up. Tetonic plates 'float'.