It has to do with the different kinds of rocks in the Earth's crust: continents are made of granite, and ocean crust is made of basalt. The more granite there is, the more continent there is, and the more of the Earth's surface rises above the ocean basins. Water fills the basins. Right now, there is more basin than continent, and so more ocean surface than land surface.

The surface of the Earth is covered about 70% by water and it may seem like most of Earth is composed of water, but actually it's a much smaller amount of water compared to the rocky planet - about 0.5% of the whole entire planet is water.

Water is the most important resource on Earth, and more importantly it sustains life. Although 75% of Earth is covered in water, Earth is mostly made of rock. In terms of weight, only 0.5% of Earth's weight is water. It looks like there is more water because water pools in low and deep basins, like lakes, streams, and oceans.