That’s been an issue for ever. I can only assume it’s a really hard problem to make the install location configurable. Plus, folks like me who don’t use Mac really don’t care. It’s a perfectly fine place to it.

EDIT: LatticeAnimal answered it. You can’t use the cache with a custom directory.

And it has to be global because the whole point is that there’s only one place and all derivations are shared between user accounts. If everyone had a ~/.nix, hard drives would explode on multi-user systems.