The reason lies in the water cycle. The total quantity of water in the lakes, rivers and oceans of the world remains almost the same always; only the distribution changes periodically. The water cycle begins as the Sun evaporates water from oceans, lakes, rivers and soil. The vapour forms clouds which precipitate as rain or snow, 75 per cent of which falls on the oceans. The remainder falls on land. Some of this water seeps into the earth as groundwater while some is returned to the oceans by the rivers. Thus there is constant balance between water flowing into the oceans and water evaporating from them. That is why the oceans do not overflow.
