That's an exception, for several reasons. The US government isn't going away any time soon, and can't just die or go bankrupt like a normal customer. Also, the government has an interest in keeping multiple contractors competing with each other, so they'll intentionally spread work out among competing contractors to keep them all busy and in business. It's just not like any other company.
Also, defense contractors frequently have multiple customers, in a way. First, many contractors sell their stuff to foreign customers (after getting approval from the USG of course, frequently with degraded capabilities). Lots of countries are buying the F-35 for instance. Also, contractors frequently have multiple customers within the government. They might sell to the Air Force and the Army, for instance, or the CIA and the NSA. Those are all different agencies, and act like different customers, without that much coordination between them, and each has its own funding and does its own contracts with contractors.
