Originally Posted by TBG RomeFreak
For the cost though? At the right price everything can be reasonable.
No it isn't.
1. This game is balanced in part around limited slot space and an eventual limit to snowballing.
2. A strong armor+magic resistance item could make carry itemization too easy. With only the standard defensive slot a carry player could greatly increase the effective benefits they are getting from lifesteal (the health lasts much longer).
3. Defensive itemization is designed to be powerful, but require different pieces and horizontal building to effectively build up. This oversimplifies it (which is usually a bad thing because carries benefit too much from over simplified and powerful defensive items). I used to be one of the players asking for exactly the type of item suggested in this post before I started to understand to thoroughly how item mechanics actually work.
4. This type of suggestion almost always turns into some kind of stackable stat stick .Riot doesn't make stat sticks unless they have to for itemization to work, and only ranged ad carries really have stat sticks at this point because they would be broken with too many diverse utilitarian items.
5. Armor+magic resistance in a single item could potentially be too strong early.
6. We don't need more Aegis of the Legion-like passives. Xypherous mentioned in the item suggestion thread that stacking such items would be very strong, but the effects would not be easily noticed by the other team. This results in games where a team loses due to a potentially op increase in free defenses on the opposing squishy champion, and for the losing team it would be unclear why they lost. This is likely the main reason why Aegis of the Legion is limited to a mid-tier item, and based on its patch history the Aura of Aegis of the Legion has been nerfed a lot culminating in a remake of the item. Clearly the item was broken with very high aura numbers.