Overqualified employees are a waste
Having hired overqualified individuals, I can attest to some of the challenges and waste of resources. Overqualified individuals tend to get bored. That is a nice way to say they under perform. They find that the work they are doing is either beneath them or not where they want to be in their career. So they find it hard to motivate themselves to perform well. They tend to go through the motions to achieve bare minimum.
The other negative of hiring someone who is overqualified is that they will leave as soon as something "better" is offered. In this case the employer wasted time in the hiring and training process. When the overqualified employee leaves quickly, the employer is back to square one.
The last thing is that some overqualified employees try desperately to change the scope and responsibilities of their work so that it is closer to their career goal / path. They focus on what they like and tend to let the other things slip.
Eventually the overqualified employee will quit to do something "better" or get fired for poor performance. It may seem like over-qualified candidates are always a bad move. Not true...the always part that is. There are occasions when they work hard and become fast trackers to success. These are the real professionals who are not selfish and are keepers. The odds are not in the employer's favor, so employees and employers do both of yourselves a favor and match the skills and career path to the position.