system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do employers reject applicants on the basis that they are overqualified?

I mean wouldn't that be a good thing?
gpt
The hope when hiring someone is that you will find a dedicated worker of an appropriate skill level, who will then get the opportunity to be promoted as their skills improve over time - ideally staying with the company long term allowing them to have a better understanding of the companies practises and systems, and removing the need to be hiring as many staff (which takes time, money and effort).

To hire someone overqualified, means they are likely accepting a job below the grade they actually want but have not yet been able to find, so are much more likely to view this job as a short term stand-in and leave to a preferable job as soon as they can.
If you have a decent engineering degree, you probably won't want to last out your time as a manual laborer in a lumber yard if you could use that job to keep you adjust for the moment until something better appears, while if you have no certification beyond high school, that job in a lumber yard is a better sounding prospect as it gives you the chance to increase your sales and management skills over time and incremental promotions.