I agree with the apples/lawnmovers analogy. MDF is used BECAUSE it has many advantages over solid wood, chief among them being its material uniformity (no knots or density variations) which contribute to its highly predictable and favorable acoustic properties.

MDF is an amalgam of adhesive resin and wood fibers bound together under intense pressure and heat. Here's a short page about it. It is made from wood, but at the same time, it is NOT wood.