My guess is that it's to try and keep us sharp in trying to obtain our goals and to motivate us to reach further for our goals. I've been lucid a few times in a storyline dream and I decided to follow the storyline because it interested me. Now, if I were non-lucid, the dream would have ended earlier. But thanks to my lucidity, I could hold on to what was happening and even move faster with a bit of dream control here and there. I dunno, maybe it can be motivation to go lucid for people too. To have some closure on those storylines. Just a thought.