Why does Thornmail do magic damage?
Between all the MR options available on AD champs it's hard to get any mileage out of Thornmail anymore. I actually lost a game earlier as Darius because my Thornmail popped a Maw as I was mid-dunk on the enemy ADC and cost me my reset and the fight.
Would it be too oppressive if it did physical damage or is there some other reason it's magic damage? Legitimately want to know the answer to this question, not looking for long debates about Maw, EoN, or Mercurial being "op".
EDIT: Not sure why you're downvoting a question, but okay Boards.

vs ad comps for more then their kits, cuz they can get
which makes those items do more magic damage
works