Because the solution only contains "Na"^(+)Na+ and "Cl"^(-)Cl− ions, and water, and not the metal "Na"(s)Na(s). You cannot evaporate the water from the solution and hope to obtain anything but "NaCl"(s)NaCl(s).
"Na"(s)Na(s) has an electron configuration of 1s^2 2s^2 2p^6 3s^11s22s22p63s1, but "Na"^(+)(aq)Na+(aq) has an electron configuration of 1s^2 2s^2 2p^61s22s22p6. That means they are not the same element, and thus, there is no straightforward way of extracting "Na"(s)Na(s) from a "NaCl"(aq)NaCl(aq) solution.
(crudely speaking, you would have to supply an electron for every atom of "Na"^(+)(aq)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"Na atoms, which is even harder.)