Now you need two expensive machines, not just one. Billet is not that expensive. I've seen people suggest this before and I looked on their site. They show a multi axis CNC machine making the parts. You would need exactly the same machine to post machine the 3D printed part, but now you have a second expensive and slow process to manage (the metal 3D printing). With billet a huge bar of material shows up, you slice each piece in a few minutes, mount it in your CNC and go. 3D printing feed powders have their own expensive processes they need to go through, so they probably cost much more than billet, reducing any potential cost savings on the material side. And the second expensive machine and process adds a lot of new expenses. Plus, you have to validate the product all over again instead of just selling them.
I bet the high price is mostly due to patents and lack of competition. It is probably not inherently super expensive to make, beyond the need for a high end multi axis CNC machine.