Here's why:

* Vastness: Oceans cover about 71% of the Earth's surface, holding roughly 96.5% of all Earth's water.

* Evaporation: The sun's heat evaporates water from the ocean surface, turning it into water vapor.

* Continuous Cycle: This water vapor rises into the atmosphere, eventually condensing into clouds and falling back to Earth as precipitation, replenishing rivers, lakes, and groundwater.

While other sources like rivers, lakes, and groundwater contribute to the cycle, the ocean's sheer size and volume make it the dominant source of water.