system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does it hurt someone to be over-qualified for a job?

I'm planning to get my PhD, but I'm not entirely sure what I want to do for a career.  I'm worried that having a PhD would make me overqualified for other jobs.  I've often heard that this happens, but I do not understand why.  If I have a PhD in one subject, but pursue a career in an unrelated field, will my PhD hinder my chances of getting that job?
gpt
The employer may feel that your high qualifications mean you won't be satisfied with the job, perhaps due to it being insufficiently stimulating. This might lead to you looking for other jobs and quickly quitting the old one.

The employer wants to avoid hiring an employee who will leave shortly, because then they would have to go through the application/interview procedure again which wastes time (edit: oh, and money, of course). So they don't hire someone that they think will want to leave very quickly.