The speed of EM signal in copper is roughly 60% of the speed of light. You also have to account for timing jitter and wait until you are sure that everybody has

The speed of EM signal in copper is roughly 60% of the speed of light. You also have to account for timing jitter and wait until you are sure that everybody has the signal to prevent going out of sync. This means that reliable distance from a single clock is just a fraction of what the speed of signal theoretically allows. Clock distribution networks use local clocks to buffer and amplify the global clock but they take a significant amount of chip area and make the chip larger. Clock distribution circuitry draws a significant amount of power. It can be 30-40% of the power usage. You want to use them as little as possible.