1. Waste of engineer time fiddling about to get things to print right.
2. Often people have to alter settings or material to suit a part, then we tend to never know what settings were actually used therefore we cannot replicate the same part reliably unless we add additional documentation/burden to the process.
3. If the spec hasn't been sent out to a third party fabrication house then we aren't sure the spec is good. Using third party makes the spec de-facto validated and our interface to third parties standardized, thus supply chain becomes redundant.
4. We get free upgrades as industry iterates, with no capital down. It's like cloud services for hardware, except safer because it's transaction oriented with no technical debt or dependencies tying us to running instances.
5. If we need finishes, materials or processes that are expensive to acquire in a printer, instead of buying new gear or obsessing over integration we can just use a more expensive outsourced process.
6. A 3D file is possible to duplicate, view and diff remotely, a custom made physical part with unknown settings/material isn't. I've actually lobbied Github to enhance visual diff for 3D files, and they're considering it internally.
Already we have used many processes: CNC machining, milling, laser cutting, sheet metal forming, thermoforming, 3D printing, injection molding, blow molding, various coatings/treatments, etc. Would hate to have to fork for all that hardware! Luckily, we don't. Also, we've just moved offices for the second time. It would cost many times more to move and we'd need many times more space if we had fabrication equipment in-house.