I asked my son (who plays violin) if he was able to practice “just for fun” and be playful. He said he couldn’t. I think some of the factors that discourage kids from practicing are also the ones that discourage adults. Kids get bored of the “same old songs”, which they are often on for weeks or months at a time. They simply view it as an extension of homework.
Another reason some kids don’t like practice is they feel bad when they make mistakes. If they have a harsh teacher, then the spirit of that teacher invades their practice mind, too. Of course, part of it is just youthful impatience and the lack of knowledge about when they “know” a piece. Perhaps they know something we don’t. Certainly their level is not up to what adults would consider mastering a piece. But, perhaps they do in fact know it well enough to move on for now. They don’t know what they don’t know, and perhaps that should be OK.[*]
[*] What I’m saying here shouldnotbe construed as condoning sloppiness or lack of care. Rather, we should be aware of when the “best is the enemy of the good”.