In an acoustic instrument, the tone depends very much on the resonances not only of the material the instrument is made from, but also on the size of the air cavities inside, and also the size and position of the holes connecting the inside to the outside - i.e. the rose on a guitar.
The structure of the instrument also has to be strong enough to withstand the tension of the strings, and any other mistreatment that it might receive while being played or carried around! And of course it has to be physically possible for a "normal sized" human to actually play it.
So in the end, the question "why does a classical guitar look the shape it is" amounts to "why does a guitar sound like a guitar" - which is probably unanswerable! In fact the earliest guitars often had quite small bodies, short necks, fewer strings (e.g only 4) and relatively few frets (sometimes only 7 or 8) compared with the later "Spanish guitar" design.
Of course modern materials and construction techniques can solve the mechanical and acoustic design problems in ways that were not practical using only the materials and hand tools that were available several hundred years ago.