Here's why:

* Self-contained: Cells are the smallest units capable of carrying out all the processes essential for life, such as metabolism, reproduction, and response to stimuli.

* Building blocks: All living organisms are made up of one or more cells.

* Diversity: There are many types of cells, each with its unique structure and function, but they all share fundamental characteristics.

While smaller components like molecules and organelles are essential for cellular function, they cannot exist or operate independently. The cell itself is the smallest entity that can be considered truly alive and capable of sustaining itself.