Here's why:

* Cells are the fundamental building blocks of all living organisms. They carry out the basic processes of life, such as metabolism, growth, and reproduction.

* They are self-contained units, meaning they can function independently to a certain extent.

* They are organized into tissues, organs, and organ systems to create a complex, functioning organism.

While there are smaller components within a cell (like organelles, molecules, and atoms), these components are not considered independent units of life like cells are.