A PhD can get a much higher paying (and likely less physically difficult) job than fast food.
That maybe be true, but they may be trying a different career, or unable to find a job in their field because of oversaturation, or whatever other reason.
The unspoken assumption when someone is "overqualified" is that they will take a better job if the opportunity presents itself.
Isn't that everybody though? If a cashier in fastfood got an opportunity to become a highly-paid streamer, they'd quit their fastfood job immediately too. But I do get your point: better credentials mean better job chances, mean greater likelihood of moving a job that's paid better.