Why is the value of volume charge density $ρ$ zero in lossy medium?

Given is the page of a book which I was studying. I was trying to study the derivation of the wave equation in a lossy medium. As I have underlined a sentence that says, "most of the case in which wave is propagating does not contain the charges". Why is it so?

This is not true in all cases, but at least approximately valid in most lossy media. Note that even if $$\rho\ne0$$, it is only the gradient of the charge density ($$\vec\nabla\rho$$) that appears in the wave equation, so the wave propagation is unaffected by a uniform non-zero charge density. Even if $$\rho\ne0$$, its gradient is usually (but not always) small.