Skilled or not, it's a VERY HEAVY occupation. Mainly in my neighbourhood, since the mailman has to climb a lot of stairs. Add up a heavy bag with all the mail and it's not hard to imagine why they retire at 55: their knees simply can't go on anymore. And to be able to retire at 55, you need to have a better pension then people that retire at 65. And in order to build up a better pension, you need more wages.
On the other hand a small suggestion to ease the burden for our mailmen and -women (other then something that takes just higher wages): all mailboxes need to be on ground level and something like this:
or this