Why we can't use runesmith on uniques?

Entire point of league - shipments farm, and runes and other stuff in return.

Why runesmithing only allowed to use on rare gear?

I don't get...

If you play with unique weapon - you are basically playing standart. Runes are the only new thing here and if you can use it - gg. No new map interactions, no new ways to build your character. Only unrewarding new bosses that you dont get to see and trash from shipping.