Hi! Disposable means "available/ready for use" and workforce means "people who do all the work", so a disposable workforce means people who are available to do/finish a project. The sentence means that every great civilization was built because of the availability of the people (workforce) who did all the hard work.

Workers who can be fired at will. Or workers that can be used for a task (say, building a specific bridge) and then be fired once their use has ended.