Boeing is a defense contractor that has well-developed supply chains. If we got into a shooty war and needed more airplanes, Boeing can build them. You don't want to be caught going into a shooty war
without the ability to reinforce your equipment. And if you do, you can convert the commercial manufacturing capacity to military to much greater effect if the commercial side is already at super-scale. The capability is worth the subsidies IMO.
Boeing also ensures American interests dominate the skies around the world. There just aren't that many manufacturers of aircraft, and Boeing spreads American influence / values across the globe as an essential part of infrastructure. Airbus does the same for the EU.