Even that's not wholly true. Roughly half of the country, from Denver to about 250 miles from the Pacific Ocean, are either mountains or arid deserts that support only very minimal farming and livestock, most of that area is nearly uninhabited (density under 10 people per square mile). You get extractive activities in the Plains States, but that accounts for perhaps 8% of GDP. The same holds for much of Appalachia.
Most food is grown within 100 miles of urban centers because it costs money to ship grain, produce, meat, and poultry or raw ore to where it's processed, and most of that processing is within that 100-mile limit. Most cities could sustain themselves quite nicely if they claimed everything within that perimeter - they just wouldn't necessarily have as much for interstate commerce or export. That's even true in the Midwest.
What's more, extractive industries and farming do not actually generate all that much GDP by itself. Most of that gets added in as processing, packaging, shipping, and marketing, along with infrastructure support such as tractors or heavy trucks, all of which are generally produced in the industrial areas of cities.
The EC (and winner take all apportionment) skews this even more, as it overcompensates for essentially unproductive land at the expense of productive talent.
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