The short (but possibly not satisfying) answer is that Homebrew's relocation of packages (including binary relocation) is best effort, in part because of the myriad ways in which packages can embed absolute (or incorrect relative) paths and other state in their build products. macOS bottles are generally more relocatable (in part because of a lot of scar tissue around binary relocation), but it's a general problem with build system quality, build complexities, and - reasonably - disinterested upstreams.