If I understand correctly, flat earthers have to believe the entirety of anything having to do with space has to be fake, in order for the flat earth ideas to survive. This means, we never explored the moon and the moon landings are all faked, right?
So, why would we invent the Van Allen radiation belts? These belts are one of the reasons why moon landing deniers say we did not go to the moon, we could not pass through the belts and live. It's not true, because geometry. But, why invent them if space itself is not real?
I don't know all the reasons why people say we did not land on the moon, but if you are a flat earther, the flat earth and it's many issues are enough of an argument to agree that the moon landings were faked, and all the science "around" the globe earth is also fake.
So, why would anyone invent the Van Allen radiation belts? Flat earthers aren't trying to show why the moon landings were faked. If there's no need to explain, why would something that makes travel to the moon more difficult have been invented as part of the fake moon landing story? It's an unnecessary complication.