I mean, they are fun, but seriously.
I agree that stoking kids' (and adults') imagination is useful and has benefits. But it sounds like you're claiming they literally won't be able to conceptualize anything outside of reality without them.
If a kid is willing to take hypothetical scenarios seriously in a story I'd be willing to bet money they're more willing to take other hypotheticals seriously too. Moral reasoning tends to require hypothetical scenarios. If the other party isn't willing to engage in them then you can't make an argument based on them.
I'm simply saying that these stories help you take what if scenarios more seriously.
There is quite a massive distance between "You can take some lessons about racism from this content" and "If you do not consume this particular content you will not understand racism."
Imagine you were trying to have a discussion about racism with someone. You would tell them "How would you feel if one day you woke up black and people were biased against you due to your skin color?" They would tell you that it's the dumbest thing they've ever heard. Who have you ever known that woke up with a different skin color?[0] For someone to take an argument like this seriously they have to be willing to engage in a hypothetical scenario. Waking up with a different skin color is as fantastical as the stories you're talking about. This means that they won't engage with your hypothetical scenario.
[0] This is an argument made by James Flynn in his amazing TED talk: https://www.youtube.com/watch?v=9vpqilhW9uI