It is far easier to source a text than to gather sources and write a new article from scratch. Sourcing is grunt work where writing a new article requires creativity and whatnot.
That kind of thinking is one of the biggest problem with Wikipedia at the moment. Sourcing should always come *before* writing. The source is where the information came from, that's what "source" means, so you have to have the source before you can write the article. Adding sources afterwards is a way of fixing a problem - unsourced articles - it should not be a part of the standard process of writing articles. The problem should never be created in the first place.