alt Hacker NewsUUIDv7 didn't mature until long after the Go standard library was mostly settled. By that point, there was already an idiomatic 3p dep for UUIDs (the Google package), and as you can see from the thread, there were arguments in favor of keeping it 3p (it can be updated on an arbitrary cadence, unlike the stdlib).
They could have implemented the other types of uuid generation, as well as having the standard type. Then evolve.
UUIDs rarely get new versions. I don’t think it’d be too much to expect Go to stay relatively current on that.