I agree, but I think they remain in use just because most people happen to know their own zip code (from using it in mailing addresses). There is an alternate

I agree, but I think they remain in use just because most people happen to know their own zip code (from using it in mailing addresses). There is an alternate system that's completely public-domain, maintained by a proper government agency, and actually intended for spatial division of U.S. territory in a way that's useful for grouping populations: the U.S. Census Bureau's census tracts. However these are somewhat newer (they didn't cover the whole U.S. until the 2000 census), people don't have their own census tract # memorized off the top of their head, and there seems to be less infrastructure & software built to use them outside of GIS people using actual census data.