The speed of light in fiber (or electrical signal in copper) is less than the speed of light in vacuum. There are delays (very small) converting a signal from

The speed of light in fiber (or electrical signal in copper) is less than the speed of light in vacuum. There are delays (very small) converting a signal from electrical on one end of the fiber to light and back to fiber on the other end. For this reason, DAC tends to have measurably lower latency compared to fiber for in-rack networking. The length of an undersea cable is greater than both the straight line and the great circle distance between two points on the earth's surface. These things do not explain all (probably not even most) of the difference between the latency you suggested and that in the real world, but I hope they help to suggest why the naïve calculation is not achievable.