That's because a corporation does not have to commit to a certain level of production. On the other hand, the government does so all the time at the request of the people. And when you guarantee output X for whatever good/service, the notion of full employment makes sense. As a representative example, it takes about 5,000 police officers to keep the peace for 1,000,000 people in an urban environment (rough estimate, just an example).

Of course, what goods/services should be guaranteed and what the value of X should be for each one is and will remain an open problem.