Its easier to differentiate between average item level averages when the numbers have decent gap between them. If you have 5 pieces of gear with item levels 70, 60, 60, 50 and 50, your ilv average is 56. 5 pieces of gear with 90, 90, 90, 60, 60 gives an average of 78. You can set better thresholds that way. If ilvs were just 54, 53, 51, 51, 50, the average is 51 rounded up. 5 pieces of gear with 54, 54, 54, 51, 51 gives an average of 53 rounded up. When you do the ilv checks in duty finder its better to have thresholds of ilv60+ or ilv70+ average. When significantly better gear results in just 2 points of difference on average when ilvs go up by just 1 instead of 10 it allows for much less freedom.

In the end though, it really is just numbers. Bigger numbers actually offer more variation and room for separation. MMO players want to see improvement. And seeing a new piece of gear letting you do 10 extra points of damage per hit is way more satisfying than 1 extra point of damage per hit.