I'm also wondering the answer to this question. To my understanding, go-crypto is actually maintained separately from the Go stdlib by the Go developers[0]. So why isn't this just a fork of crypto? Why fork the entire language? Why can't this be upstreamed?
This is a library of supplemental crypto algorithms. You won't see things like AES, SHA-2, or the NIST curves in here. Those things are part of the standard library in Go.
[0]: https://github.com/golang/crypto