Because there are so few of them in existence. A violin is by no means the simple instrument it looks. It took centuries to ascertain the best materials, shape and proportions for it. A soft wood is used for the front or belly, and a hard wood (usually maple) for the back and sides.
A very slight difference in the thickness of any part, or in the curves of the front or back, or in the depth of the body, or in the position of the little sound-post inside, connecting front with back, or in the shape of the f-holes in the front, or in the quality of the varnish used, may have a great influence on the tone. The violins made by Antonio Stradivari, of Cremona, during the first quarter of the eighteenth century, are still the best ever produced; and fetch huge sums. Some say that their remarkable quality is due to the special varnish prepared and used by Stradivari, but it may be partly due to the mellowing influence of time.