Besides which those computers may well come with software different from what the Army needs.
I've worked for one huge project (1800 desktops, 200 or so servers.
They were delivered with Windows 95 and Office 95 Std. (this was 1998).
The company wanted Windows NT4 and Office 95 Pro..
The supplier told us (and no reason to not believe them in this) that the extra work needed to remove the software from the machines and change the packaging to remove the CDs would be more expensive than the money saved (the importer got the machines boxed and preinstalled from the factory elsewhere in the world. They'd each have to be unpacked, formatted, repacked without the CDs, those CDs shipped back to the factory, all of course at our cost).
So we just contacted Microsoft for a site license for 2000 machines (some future expansion and spares) for Windows NT4 and Office 97Pro.
If the same were to happen again today the manufacturer would likely have an option of an empty machine as standard of course, but at the time there was little demand for that.