The advantage of 3D printing comes with making very complex shapes. As far as I know, most parts in a car are pretty straightforward (gears, shafts, pistons etc.). Am I off-base?
The problem with metal fabrication is that a good CNC machine is at least $50,000 and more normally $250,000 or so. Combine the fact that the shop owner wants that machine running 24/7 to make bank, the fact that an error in the steel can cost you $20,000 in raw materials, and the fact that a single programming bug can damage the machine and you have a recipe for very constrained, conservative usage.
Contrast this to 3D printers: "Huh. That sucked. I'll print it again." Material lost: Under $1000. Machine damage: Nil. So, you'll print all manner of crap until you get it right.