In most cases the performance penalty of having an extra internal array to keep track of insertion order is minimal, and the whole point of built-in collections is so people can quickly write correct programs. When optimizing for performance default collections are likely to get replaced with hand-rolled versions anyway. But in all other cases a dictionary that “just works” is preferable to one that has such an annoying footgun that the go team had to randomize the iteration order in an attempt to treat the symptom instead of choosing correctness. Go isn’t even a high-performance language and many language design choices (channels!) explicit prioritize correctness over performance.

It’s like having an unstable sort as the default standard library sort function. People reasonably expect that when calling sort twice the second sort to do nothing, but you can always find people who will passionately argue that people deserve to get burned if they assume a sort function is stable.