The std lib is not the paragon of go virtue as some people make it out to be. The maintainers have been frank with some of the mistakes and things they'd do differently now but because they place the Go 1 promise above everything else (a good thing), it has some patterns not many would recommend these days, error handling among them.
And it's not like they couldn't add better types to errors without breaking backwards compat. Returning a type that implements error instead of fmt.Errorf doesn't break existing semantics.
