Why does Duskblade's passive deal true damage?

First reason why it doesn't really make a lot of sense is because you'll must likely get Duskblade in a penetration stacking build, which means that all the penetration you have is wasted on the true damage, since true damage takes armor out of the equasion completely.

Then there's the fact that true damage is something you want to have against armor stacking targets like tanks and bruisers while flat armor penetration / lethality is something you want to get against targets that DON'T stack armor, so basically squishy carrries, because of the way diminishing returns work.

I think it would make much more sense if it would deal physical damage which would make it fit better in a lethatlity/ armor penetration build. Of course the flat values would have to be higher than right now to make up for the fact that it wouldn't be true damage anymore.

What this would mean is that it would be less effective against bruisers and tanks which the assassins who build it don't really want to deal with in the first place anyway and much more effective against squishy carries, especially marksmen.

I think it might just need that change because most high elo players don't seem to build it, even on champions like Kha'Zix which in theory should be one of its ideal users.