There's a reason for it, which is that set union and intersection (types are just sets of properties) don't have unique inverses. (A∪B)∪B = A∪B. What would the inverse of ∪B be? One could argue for A or A∪B, but only A∪B is ⊇ A ∀ A, B; hence, the only general solution.
I'm still not sure how easy it will be to remove one type from a union. That would be a gamechanger in error-handling ergonomics.
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