Here's why:

* Viruses are not cells. They are much simpler structures, essentially consisting of genetic material (DNA or RNA) encased in a protein coat called a capsid. Some viruses also have an outer envelope derived from the host cell membrane.

* Cell membranes are essential for cells. They regulate the passage of molecules in and out of the cell and are crucial for maintaining the cell's internal environment.

* Viruses rely on host cells. They lack the machinery to replicate on their own, instead hijacking the host cell's machinery to produce more virus particles.

While some viruses have an outer envelope, this is not a true cell membrane. It's a modified fragment of the host cell's membrane, acquired as the virus buds from the cell.