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.)
