Why can I see the diffuse lighting effect on a perfect mirror?
Dielectric materials (which is what you get when metalness is 0) don't exhibit a mirror-like effect. Think of a sheet of smooth, non-transparent plastic. Real-life mirrors are panes of glass or transparent plastic covered with a thin layer of metal. Try a white base colour (1,1,1) and full metalness (1) instead.

As Hubble pointed out in his comment, Unity won't do full ray tracing for you out of the box, but it should be able to do some approximation with screen-space reflections and local reflection probes.