Because they're really different? system-ui is the specific font the OS uses for its interface... San Francisco in macOS/iOS/iPad OS, Roboto in Android, Segoe U

Because they're really different? system-ui is the specific font the OS uses for its interface... San Francisco in macOS/iOS/iPad OS, Roboto in Android, Segoe UI in Windows, etc. sans-serif is often Arial or Helvetica, but could be anything.