Here's why:

* Viruses are much simpler than cells. They are essentially packages of genetic material (DNA or RNA) wrapped in a protein coat.

* Viruses lack the necessary components for independent life. They cannot reproduce on their own and require a host cell to replicate.

* Viruses don't have their own metabolism. They don't produce energy or carry out other essential life functions.

Therefore, viruses are considered non-cellular entities.