Items?

I was looking at items and noticed armour and mr damage items. Champions that are ad have many options for mr and damage like mercurial scimitar malmortius and they are capable of running mercury treads or because ad damage means autos and abilities hurt. While ap champs have zhonyas and athenes. Magic resist shield plus a cleanse and boots with reduce cc cd totally beats an immobile stasis and healing ppl with shield. Just seems a bit unfair that ad can scale better than ap and can take defensive items and still have damage. Ap feels like it can be negated better than ad