They fit into the ballad form, they express a certain sentiment, they are paradoxically humorous, and so they have been copied and pasted from song to song. Such imitation is common in oral literature. Indeed, it relates to how literary genres are formed. Like folk literature, web design lends itself well to imitation. Pure HTML design is a limited medium whereupon a million themes can be spun from the same set of tags.
Web designers imitate each other's work both in terms of content and design.
We find a site with an effect we like, we peek at the source code, and we think about how we can integrate the effect into our own work. We might even copy a snippet of code for future use. Or, in order to understand layout, we might copy an entire document, put it into our text editor, slap a border around the tables, view it in our browser, and proceed to deconstruct and tweak. But thievery can be blatant. This sort of thievery is easily identifiable and tends to infuriate the original author.