No its because 99% of the time people use enums to give names to magic constants... That is it. Go went for simplicity and const+iota achieves it just fine. People act like enums make or break software itself or something.
That seems unlikely to me to be the actual explanation. It could very well be what you prefer or how you would do it, but I can definitely assure you that the Go/other infrastructure teams think about these problems and hear plenty of complaints about lack of union type support.
Yea but with a tiny bit more effort they could have ensured that an invalid value is never assigned to an enum, iterate over the values, ensure switch statements handle every case, etc.