Uh, grabbing variables out of the current scope to format your string is most certainly magical. It's also way less explicit than actually passing variables into a formatting function.
It might be a little harder to read, but that's a lot different than explicit.
fmt.Sprintf("Hello %s!", username) is very explicitly using the username variable from the local scope, and nothing but the username variable can ever get included in the output string. At most, a user could put a %s in their string, and get the username to appear somewhere else in the output... but they wouldn't be revealing data that wasn't already intended to be printed out.
In comparison, interpolation is opening a door to let anyone extract whatever variables happen to be in scope at the time by putting #{password} or #{secret_key} in their string. By moving the definition of what variables get printed out into the data, you're opening a really big hole in your code... it also makes it a lot harder for the compiler to check for correctness.
