While I agree with you that anonymous sum types are useful, the language designers tend to avoid anything that might include a hidden cost (in this case either the size of the enum or cost of boxing). Something along those lines might end up in the language eventually, though.

The language wouldn't have lost much without tuples actually. But they do at least have direct pattern destructuring utility. Anonymous enums don't.

They also don't pose potential future awkwardness if/when variant types become a thing.