Answer
Roots and stems keep growing to seek out more for the plant (water and minerals in the former case and sunlight for photosynthesis in the latter), while leaves are meant to absorb light and do a job in the place where they first appear. Further, if they kept growing, they would interfere with one another and might also become so heavy as to fold, inhibiting their photosynthetic role.
Work Step by Step
The key here is to see that growth is part of the role of stem and root while leaves have a more limited time for growth and a long time to function physiologically.
