Big factories can be retrofitted in times of world war to build tanks/guns/etc. It's seen as a national security thing.
1. There is the problem of depending on potential enemies (all others have potential to be enemies) for goods that are critical to the military and/or general economy. Suppliers can threaten to cut supplies. Note that this can happen even if actual shooting war is unlikely; the threat to cut supplies is a brutal negotiating tactic that can block us diplomatically. For example, if we depended on Russia, they could demand that we support the Syrian government.
2. Lots of physical things contain computer chips. He who builds them is in control. Remember when France gave the UK some secret codes to make Argentina's missiles (purchased from France) fail. It's not just military equipment. Consider your Ethernet card and/or motherboard chipset. It could include a hardware debugger with full memory access, controlled by magic packets. What happens when we can only get those from government-owned Chinese companies? What about modern computerized farm equipment that phones home for licensing? If that were foreign, could a foreign power disable our farm equipment at a key time, causing us to face food shortages?