Because the type named error is used in that flawed way orders of magnitude more than any other type. If there were other types that were consistently used as the last return value in functions that short-cirucuited when calling other functions that retuned specific sentinels in their final value when called, there would be reason to do it for them too.

In fact, this is exactly what Rust's ? -operator already does, and something that's obscured by the oddness of using pseudo-tuples to return errors alongside non-error values rather than requiring exactly one or the other; `Result` in Rust can abstract over any two types (even the same one for success and error, if needed), and using the ?-operator will return the value from the containing function if it's wrapped by `Err` or yield it in the expression if it's wrapped by `Ok`. In Go, the equivalent would be to have the operator work on `(T, E)` where `T` and `E` could be any type, with `E` often but not always being an error. Of course, this runs into the issue of how to deal with more than two return values, but manually wrap the non-error values into a single type in order to use the operator would solve that with overall way less boilerplate than what's required currently due to it being rarely needed.