[OT] Reason for all non-user program’s priority being 0?

Hi All, Stupid question time, and really completely off topic for this group, but my search-fu didn't really find anything, and there’s a good chance someone here knows, so... Why is almost every non-user program’s priority 0 (nice at -20)? Instead of them having priorities scattered based upon system importance say between 0 and 5? Logically it just doesn’t seem that basically every priority in Nix should be either 0 (system app) or 20 (user app). Just a dumb question that’s been in the back of my brain for years. Best, Michael