What's the rationale behind Go choosing internal iteration (the iterator calls a function for each value, like Ruby) over external (the iterator returns each va

What's the rationale behind Go choosing internal iteration (the iterator calls a function for each value, like Ruby) over external (the iterator returns each value, like Python and C#)? My understanding is that internal iteration makes it easier to write iterators (producers) but harder to write the consuming code. That's why Go needs to re-write the body of each `for` loop as a function body, including special handling for `break`, `return` etc. External iteration OTOH makes it harder to write producers but easier to write consumers. Python and C# therefore allow external iterators to be written via coroutines/generators. Wouldn't Go's goroutines make the coroutine approach to external iterators straightforward? Whereas the re-writing necessary for internal iterators seems convoluted?