Can we get 12 armor instead of 10? Why do we get less armor when MR is a stronger stat?

At level 3 an average mage taking +10 armor will have around 40 total armor. At level 3 an average AD assassin taking +12 MR will have 46 total MR.

I understand that armor scales better late game but MR is objectively a stronger stat considering that magic damage comes in short long cooldown bursts while physical damage is sustained and continuous because of basic attacks. Not to mention the entire point of these resists is to reduce early-mid game game, a point before your armor has scaled to higher levels.

Lets not forget the near limitless availability of low gold cost armor penetration in the game currently. Lets also not forget the fact that magic pen is incredibly rare on kits but armor pen is on over 10 champions base kits.