I really enjoy taking psychological tests and I have some trouble empathizing with people who do not share my enthusiasm. Some people, I suspect, prefer to "wander the world blind." The Delphic oracle's injunction to Know Thyself is threatening to them, perhaps.
The Delphic oracle
If you are under 25, your Conscientiousness scores may rise as your prefrontal cortex continues to become fully myelinated. But other than that, expect stability in your scores. The best way to test the reliability of this kind of measure is to file your results and then re-take the test in a year. You will probably be a bit surprised (disappointed?) about how little your scores have changed.
The radical behaviorist John B. Watson (who didn't believe in personality traits or "personality" at all) offered this explanation for the consistency of observed behavior (and, by extension, the consistency of personality trait measurement) [emphases added]:
"After [age] 30[,] personality changes very slowly owing to the fact...that by that time most individuals, unless constantly stimulated by a new environment, are pretty well settled into a humdrum way of living. Habit patterns become set. If you have an adequate picture of the average individual at 30 you will have it with few changes for the rest of that individual's life -- as most lives are lived. A quacking, gossiping, neighbor-spying, disaster-enjoying woman of 30 will be, unless a miracle happens, the same at 40 and still the same at 60."
John B. Watson