Itemization, champion design, and masteries are really the main culprits.
In terms of itemization, you mostly need to look at the ways in which offensive items vs defensive items, and the disparity between unique offensive passives/actives and defensive ones.
Now i'm not going to list all the items with unique offensive passives and such, but i think you get the point. Now let's look at the items which have defensive unique passives.

Do you see the problem? Offensively stats are massively over represented on average in both offensive and defensive effects.
As far as champion design goes, it's more about how they make the champions scale and how much premium stats have been normalized. Things like CDR, armor pen, crit... all of these result in more damage output, and a higher standard which other champions are forced to compete with, which often results in adjustments to other items and/or champion scaling buffs.