Sure, but then you need to maintain it. I don't know about you, but I never had the discipline to update it with every package install and uninstall. It's especially annoying when you have multiple devices.

Declarative package management doesn't have that issue since you're managing the packages by editing the list.

Besides that, the home-manager approach works on any distro (and os?), you get bleeding edge packages, you get a built in rollback system, and you can handle configs as well (but I mainly just symlink them anyways).