Question about Deathfire Touch

Why does it not scale with your level?

Thunderlord's Decree does, as does Fervor of Battle. All the other capstones are just flat percentages, they are always relevant. Deathfire Touch though... it just does 8 damage. That's it.

I'm not complaining that it's too weak or too strong. I was just looking at all the capstones and was confused why if doesn't seem to scale at all. This is especially so considering that it seemed to be intended for making poking during sieges and the laning phase more painful. Instead, even the pokiest pokers use Thunderlord's instead because it not only does way more damage, but is backed up by Precision's delicious bonus penetration.

So... yea. Why does Deathfire Touch not scale with your level like all the other Capstones?