Put simply, we speak differently than we write. So, when we write a paper and then read directly from it, it typically comes across as stilted, which often makes it much harder for our listeners to follow and encourages their minds to drift. I'm not suggesting that extemporaneous speakers are better than those who follow a manuscript. In fact, many are not ("Preach Like TED?"). Rather, the trick appears to be translating the written word into the spoken word. Unfortunately, quite a few of us academics have yet to master that talent.
Note: To be clear, I make no claim to have mastered this talent.