Here's why:

* Passive transport relies on the natural movement of molecules from an area of high concentration to an area of low concentration. This movement is driven by the concentration gradient and does not require the cell to expend energy.

Here are some examples of passive transport:

* Simple diffusion: Movement of molecules across a membrane from high to low concentration.

* Facilitated diffusion: Movement of molecules across a membrane with the help of a transport protein, but still driven by the concentration gradient.

* Osmosis: The movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration.

In contrast, active transport requires the cell to expend energy (usually in the form of ATP) to move molecules against their concentration gradient, from an area of low concentration to an area of high concentration.