How to teach theme when students keep saying "the moral of the story is be nice"

I've spent two weeks on identifying theme with my 4th graders, and I'm ready to bang my head against the wall (quite literally).... No matter what story we read, whether it's about perseverance, honesty, facing fears, whatever.... when I ask about the theme, at least half the class says "be nice to others." 🤦‍♀️

I've explicitly taught that themes are universal messages about life, provided examples, created anchor charts, everything. When we read a story about a girl overcoming stage fright through practice and courage, three students still wrote that the theme was "be nice" because "the mom was nice to her daughter." I'm starting to think I'm the world's worst teacher.

Has anyone found a breakthrough activity that helps kids understand theme at a deeper level? Or should I just accept "be nice" as their universal answer to everything?