Greater Greater Pentagon

Ryan Avent speculates about how the Pentagon site could be a lot better:

The Pentagon occupies an enormous chunk of land in close proximity to the office, residential, and retail spaces in Pentagon City, abutting the river, and directly on top of a Metro station. Which is all well and good, except that Defense feels that the best way to use much of that land is to cover it in asphalt. Somehow Commerce, Transportation, Treasury, Agriculture, State, and so on get by without acres of surface parking. Why is Defense special?