Here's why:

* Water makes up 70-85% of a cell's mass. It acts as a solvent for chemical reactions, helps maintain cell shape, and plays a crucial role in transporting nutrients and waste.

* Other cellular components, like proteins, carbohydrates, lipids, and nucleic acids, make up the remaining percentage. While these are essential for cellular function, they are present in smaller amounts compared to water.

So, while proteins and other molecules are important, water is the primary contributor to a cell's overall mass.