Injection molds need to be ridiculously precise, as in 1/1000 of an inch of can render it useless. Also, I have doubts that the layered result of 3-D printing can hold up to the extreme abuse these things get put through after tens or even hundreds of thousands of cycles. We are talking about foot thick plates of steel bending and warping from fatigue damage after a while.
Even if you could 3-D print the bulk of it, the job of polishing and detail work would still need to be done manually.
Edit: after Reading the comment below, I admit I may be somewhat biased from working with more complex and detailed parts with very right tolerances, so there may be some applications where it is feasible. All the molds I've seen/worked with were made the old fashioned way.