Incompetent buyers is the best description for it, but it's not all. There are few financial incentives/penalties in government, the only real risks are political. You'll not get in big trouble for securing funding for a project that ultimately doesn't succeed, but you will end up in big trouble if you run a successful and frugal project that steps on the toes of someone higher up.
And ultimately those political forces are what drive these projects. You do some work for a client in the army, and then Mr. Army gets on a bus with a guy in the air force. Mr. Army sings your praises to a big-shot from the Air Force and just like that you've got another $5M contract (to make Drupal websites).
