Because the solution only contains "Na"^(+) and "Cl"^(-) ions, and water, and not the metal "Na"(s). You cannot evaporate the water from the solution and hope to obtain anything but "NaCl"(s).
"Na"(s) has an electron configuration of 1s^2 2s^2 2p^6 3s^1, but "Na"^(+)(aq) has an electron configuration of 1s^2 2s^2 2p^6. That means they are not the same element, and thus, there is no straightforward way of extracting "Na"(s) from a "NaCl"(aq) solution.
(crudely speaking, you would have to supply an electron for every atom of "Na"^(+)(aq) in the solution, which is quite hard to do. You would also have to "undissolve" the particles and collect them into a lattice of metal "Na" atoms, which is even harder.)
