Here's why:

* Viruses are not cells. They are much simpler structures consisting of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. Some viruses also have an outer envelope.

* Cells, on the other hand, are complex, self-contained units of life. They have a nucleus, which contains their genetic material (DNA), as well as other organelles that perform various functions.

So, while viruses can infect cells and use the cell's machinery to replicate themselves, they lack the defining characteristic of a cell: a nucleus.