There's absolutely no guarantee that this could be made in individual pieces and assembled. You can easily make connected systems that are not possible to assemble as discrete components, using this type of deposition ("3D") printing.
This also doesn't require anyone to touch it during manufacture, until the piece is complete; limiting contaminant exposure keeps costs and turnaround times small, while also minimizing the chance for errors to creep in. Ever try assembling tiny, fiddly parts with gloves on? And to cap it off, in a clean room, where you have to pass through an airlock on every break even if it's just to the john? Or in free fall, even, as squishybrain notes? Anything that makes the process less of a pain and less error-prone is a damn good thing.