I have a family member who recently retired from working as a mail carrier for 40+ years. The generational turnover is real; he says they can't hang onto young people because they hire them as contractors and work them to the bone, and the benefits (e.g. federal pension) aren't there. I'm not sure how you can work someone to the bone who doesn't want to be worked to the bone, I can't validate the claim that they are hired as contractors, and I suspect the comment about the benefits/federal pension has more to do with how the rules have changed since he was hired. Basically, in his experience, new hires get a raw deal and don't want to stick around.
My relative would scoff at the notion that carrying mail isn't manual labor. None of the individual actions is all that hard, and the weights (up to 40 pounds IIRC) aren't that heavy in isolation, but the repetitions add up very quickly. He loved being outside as well as the autonomy he had before GPS was installed on all the trucks. Now the routes are micromanaged by a computer program.
So to hear this guy talk, the job ain't what it used to be. /anecdata