Writers like to quote other writers. Someone once complained that they really didn't see what was so great about Shakespeare, because all he had done was take a stack of famous quotations and string them together. We quote other people when when we think they have a unique way of saying something, or when the quote seems particularly apt. So why reinvent the wheel?
The Bible is full of quotations - by which I mean it is oft quoted, but not necessarily as inerrant truth. One of my favourite put-downs comes from two different places in the Bible:
"And [Judas] went and hanged himself." (Matt 27:5)
"Go, and do thou likewise." (Luke 10:37)