Here's why:
* Direct objects receive the action of a verb. The verb in this sentence is "complete," and it is followed by the object "the job."
* However, the sentence is in the infinitive form (to + verb). This means the verb "complete" is not acting on a direct object. Instead, the entire phrase "to complete the job" is acting as the subject of the sentence.
Think of it this way: The sentence is essentially saying "The act of completing the job should happen before lunch."