Comment by DCKing

It's for predictability in upgrades. Homebrew allows you to separate system packages (from apt or dnf) from user packages (from homebrew) [1]. Running apt upgrade or dnf upgrade can render your system unbootable if you're unlucky (or unstable or degraded if you're less unlucky). Running brew upgrade can at worst break some of your own user's setup or tools.

Since everybody runs their own unique permutation of apt or dnf packages, adding as little as possible will keep you as close as possible to what distro maintainers test. There's even OSes like Fedora Silverblue or Bluefin or SteamOS that ship with a fully baked _image_ - where installing system level packages is strongly discouraged - which helps ensure predictability and stable upgradeability.

Homebrew packages also tend to be more recent (this depends on your distro of course) and don't require elevated permissions to install.

[1]: Other unprivileged package managers like Mise or Nix do the same of course