Why do scientists consider the cell to be the basic unit of life?

Cells make up the smallest bits of every living thing. Everything that contributes to a living thing being alive comes from cells, making it the fundamental part of life.

Because they are responsible for keeping things alive.

Explanation:

Cells are responsible for creating organism and maintaining them. Cells constantly produce proteins, which are responsible for the structure, "communications", and creating energy. Each cell serves a different function which then produces different proteins.