I'm not sure if golang has the same fundamental issues in common use, but in e.g. C you don't want the compiler reordering your structs or adding arbitrary padding because that makes it incompatible with other in-memory representations - e.g. if you're using shared memory with another process that hasn't received the same optimizations, if you're loading raw data into memory/using mmap, etc.

Likewise, one of the examples is moving from an array of structs to a struct of arrays; that's a lot more complex of a code reorganization than you'd want a compiler doing.

It would be good to have a static analyzer that could suggest these changes, but, at least in many cases, you don't want them done automatically.