At each level, you will have a "par" value for each stat. Let's say 1 armor gives 1% damage reduction at level 1 , the relative value of 1 armor will have to decrease significantly as you level, or before long you'll wind up with 300% damage reduction(or crit, or surge and so on), which would ridiculous fast.
This actually happened in The Other MMO. In Wrath gear values inflated too fast and tanks had too much defense. So they had to implement a flat 20% reduction in chance to dodge, to help balance incoming damage.