Because the US doesn't manufacture everything it needs (entirely, from initial processing and raw materials to final product), nearly everything has to be shipped long distance from wherever it is mined, manufactured, assembled, grown, etc. Some of which is international, but a lot of it is hundreds or thousands of miles away. The development of the transcontinental interstate highway system, itself a military project during the Cold War, made this possible.
Those are all shipping-related jobs. Just-in-time logistics systems mean cities are extremely vulnerable to any interruptions in shipments. Stores, for example, grocery stores, do not stock enough for more than a few days - this is cost saving in good times, as that means less need to refrigerate goods, less spoilage, less real estate or buildings needed.
The electrical grid is extremely vulnerable to major shipping disruptions. Coal plants, which currently provide baseline power for the country, only store about a month of coal on-site to run the generators. Coal is shipped in by train from WY and MT mostly. If no trains, no grid. And other plants like natural gas fired plants require a functional grid to pressurise the natural gas so it gets to the station. Even where renewables have seen wide use, like California's distributed rooftop solar generation capacity, storage is highly lacking, and the rooftop systems don't work when the main grid is out, even to power the house, on most of these "grid tie" setups, which are mainly installed as a cost-saving measure. If the grid is out, the whole country collapses because there isn't a way to restart things it without it - control systems, etc. And everything like food storage, telecommunications, arguably the national and state government's ability to govern, relies on it. Most people do not have access to the resources (off grid solar, etc.) to sustain life without access to the grid, and without any modern technology, it's not likely that such a large population could survive at all. If you want to know more about grid vulnerability, researching solar flares and electromagnetic pulse gives you an idea of what would happen. The US government also released an "EMP Commission Report" about the latter.
Because the US doesn't manufacture everything it needs (entirely, from initial processing and raw materials to final product), nearly everything has to be shipped long distance from wherever it is mined, manufactured, assembled, grown, etc. Some of which is international, but a lot of it is hundreds or thousands of miles away. The development of the transcontinental interstate highway system, itself a military project during the Cold War, made this possible.
Those are all shipping-related jobs. Just-in-time logistics systems mean cities are extremely vulnerable to any interruptions in shipments. Stores, for example, grocery stores, do not stock enough for more than a few days - this is cost saving in good times, as that means less need to refrigerate goods, less spoilage, less real estate or buildings needed.
The electrical grid is extremely vulnerable to major shipping disruptions. Coal plants, which currently provide baseline power for the country, only store about a month of coal on-site to run the generators. Coal is shipped in by train from WY and MT mostly. If no trains, no grid. And other plants like natural gas fired plants require a functional grid to pressurise the natural gas so it gets to the station. Even where renewables have seen wide use, like California's distributed rooftop solar generation capacity, storage is highly lacking, and the rooftop systems don't work when the main grid is out, even to power the house, on most of these "grid tie" setups, which are mainly installed as a cost-saving measure. If the grid is out, the whole country collapses because there isn't a way to restart things it without it - control systems, etc. And everything like food storage, telecommunications, arguably the national and state government's ability to govern, relies on it. Most people do not have access to the resources (off grid solar, etc.) to sustain life without access to the grid, and without any modern technology, it's not likely that such a large population could survive at all. If you want to know more about grid vulnerability, researching solar flares and electromagnetic pulse gives you an idea of what would happen. The US government also released an "EMP Commission Report" about the latter.