Why do dogs hate postmen? ?

They are strangers entering the dog's territory.

Postmen do not have time to stop and be playful with a dog.  But dogs are like babies.  They do not understand, and construe the ignoring of them to be like dislike and hostility, so the dogs bark at the postmen.

It's a territorial thing. Any unrecognised visitor tends to wind them up. My GSD used to go berserk when the bell rang! However when a neighbour visited in my drive and ignored the barking he sat down looking silly. Most dogs tend to be vocal rather that attack.

Because they approach the house, but don't actually come in or visit. In a dog's mind, that's a breach of territory, so they view that person as a threat.