Here's why:

* Passive transport doesn't require energy from the cell. It relies on the natural movement of molecules from an area of high concentration to an area of low concentration.

* Osmosis is the movement of water molecules across a selectively permeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This movement is driven by the difference in water potential, not by the cell expending energy.

While there are active transport processes that involve water movement, osmosis itself is a passive process.