Is there a reason why every champion who has scaling MR has the SAME scaling MR?

What was Riot's thought process behind this exactly? Instead of potentially using armor per level and base armor balancing variables they make them both completely static across the champ roster. Every champion has 30 base MR, and if they have scaling MR, it's a growth of 1.25/level.

Just out of curiosity, what was Riot's reason for doing this? I'm sure MR could be used as a base stat to balance champions just like every other stat in the game.