The tips of the blades travel close to the speed of sound. The blades are quite difficult to manufacture as it is, it's a big challenge to add intentional weak points and/or weight by making them foldable.
I'm not saying you are wrong, but by themselves those factors aren't entirely convincing to me.
(Also, the wings need to fold and unfold repeatedly over their lifetime; the blades could just be, in theory, shipped in pieces and then assembled once).