Question about the trend towards Total AD ratios instead of Bonus AD ratios

Looking at the recent reworks/new champions I have noticed that there are a lot more spells that scale off Total AD instead of Bonus AD. I am referring to non-AA modifier spells, as AA modifiers should scale off total AD.

As an example, lets look at new Skarner Q vs Nautilus E. Both are AOE spells and one deals physical the other magical. Both are pretty short CD. If we put 1 point in each at level 1, Naut's E deals 60/wave magic damage, while Skarner's Q does (level 1 base AD)570.33=19 physical damage, which is pretty fair as Skarner Q has shorter CD, CD can be reduced by AA and deals bonus damage on 2nd Q hit. Now let's assume they both reach level 18 without leveling up those abilities or purchasing AD or AP. Nautilus E still does 60/wave magic damage, while Skarner Q does (level 18 base AD)1340.33=44 physical damage. So Skarner's Q gets more than double free improvement from levels, while Nautilus does not.

So my question is why do new champions receive ratios like this? What is design philosophy behind this? I am just a newbie and would love to be enlightened.