The 'mental age' of Reincarnators in Isekai is such a dropped and unused plot point that I don't know really know why authors even bother drawing attention to it early in the story. It just distracts people later on when you drop it.
However, I once heard someone argue (I think it was specifically directed towards Myne of Bookworm) that while the protagonist may have the memories, knowledge, and experience of an adult... they still have the physical brain of a child. And a child's physical brain is less developed then an adults and thus does not have the same capacity to reason.
Its not an Adult's Brain stuffed into a Kid, its a Kids Brain that has had an Adult stuffed into it.