“how does it make one less suited for a job such as schlepping boxes?”
Would you hire someone who had one foot out the door before their first day on the job. Typically, those who are overqualified for a position are always looking for something that fits their skill set. It is not fair to the company to hire someone who is already looking to leave. Of course, if you are the company and you wish to do chessed with your business, that is your right, however, a hiring manager whose first obligation to his employer does not typically have such leeway. From their perspective, over qualified candidates are simply not a good fit.