Here's why:

* Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This movement is driven by the concentration gradient itself, meaning it doesn't require the cell to expend energy. Think of it like a ball rolling downhill - it happens naturally due to gravity.

* ATP is the primary energy currency of cells. It's used to power a wide range of cellular processes, including active transport, muscle contraction, and protein synthesis.

Examples of processes that *do* require ATP:

* Active transport: Moving molecules against their concentration gradient, which requires energy to overcome the natural flow.

* Muscle contraction: The sliding of protein filaments requires ATP to power the movement.

* Protein synthesis: Building new proteins from amino acids requires energy to form the peptide bonds.

Let me know if you'd like more detail on any of these processes!