So you have never shared a joke a work, got home and repeated the joke to friends or family?
People like to have a laugh. Most people repeating the joke are not claiming credit for creating it. They found it funny and shared it so others could find it funny to.
Twitter gives you an easy way to share the joke while retaining attribution - the retweet.
I agree that most of the people are just sharing the joke, but a sizable lump of them are trying to create some kind of online brand which they use to get money.
People like to have a laugh. Most people repeating the joke are not claiming credit for creating it. They found it funny and shared it so others could find it funny to.