Government in its infinite wisdom, mostly highly regulates parts that are most easily produced in plastic on a 3d printer.

You can print Glock frames and AR lowers, and finish the guns with solid steel, unregulated parts.

For the FGC-9 you can produce your own rifled barrel at home without expensive tools.

But yeah, guns that are totally 3d printed (nobody does this except as a political statement) are more dangerous to the operator than the target.