Because those types are not object types, so they don't have methods associated with them at all. This is unlike, say, CLR languages in which all types are object types.
There's been research on modular implicits for OCaml to solve this more generally, but that's not landing upstream anytime soon.
There's been research on modular implicits for OCaml to solve this more generally, but that's not landing upstream anytime soon.