Because I don't believe in private methods in Ruby, is the short answer :)

(I don't see method visibility as useful in Ruby (`object.__send__ :private_stuff` and here it goes...), at all, and quite possibly even in OOP in general. The `__` Python-like notation simply hints the user that this method is somehow "weird", and that is emphasized with the `@api` YARD notations consistently used in the source code annotations. It is definitely preferred to the `private` expression, which is not used anywhere in the codebase I believe.)