You could, but then you still have the original data in the original location which needs to be scrubbed. And go does not, to my knowledge, provide semantics that allow for doing this in a way that should be considered cryptographically reliable.

The go runtime might optimize away your memzero, or it could have created other copies that you don't have a handle to.

In the Rust version of my library (and maybe in the go version, it's been ages since I worked on it), I go out of my way to make it difficult to copy data from runtime-managed memory into a secret buffer. You can do this, and it makes a best-effort attempt at zeroing the data when you do, but you lose a lot of hard guarantees when you do.