Since humans already have the biological machinery to grow complete humans from scratch in 9 months, why not utilise this capacity in its downtime to commercially grow single body parts?

I believe they are already working on specific parts manufacture without needing a womb.

Sounds like the Island movie 2005 trailer

BBC – August 27, 2004 – New jaw grown on patient’s back – “University of Kiel researchers “grew” a replacement jaw-bone in a muscle in the patient’s back and grafted it in place.”

They are doing that already. the needed body part is implanted under the skin of the person needing it so it grows & generally becomes more compatible with the patient. With head injuries, they remove a section of the skull to relieve the pressure & then implant the skull section in the patient’s body in order to keep it viable until they can replace the piece of the skull where it belongs.

Like you said, “Since humans already have the biological machinery to grow complete humans…” The technology doesn’t exist to create just that one desired part. You would have to create the whole thing, cut off what you didn’t need and throw the rest away. A lot of time, effort and expense to throw away so much. And throw it away is what you would have to do since the body that was grown and the body the part was grown for would not be compatible with any other body needing a part.

They are building heart valves for research, so they don’t have to use weakened humans as trial cases.