Because the ocean is on average only 2 miles deep. You will need an additional 5 miles of water to reach top of everest. That's 2.5 time additional volume just from depth considerations. Add to that the ocean basin covers only 70% of the earth surface, where as a global flood will need to cover 100% of the earth surface, which is to say it needs to have about 1.5 x 2.5 times as much water to flood the globe to a depth sufficient to submerge summit of everest.
You can make simplifying thin shell assumptions in calculating water volume, and assume land on average has elevation close to unflooded sea level, both of which reduce accuracy by negligible amounts.
You can make simplifying thin shell assumptions in calculating water volume, and assume land on average has elevation close to unflooded sea level, both of which reduce accuracy by negligible amounts.
