Legend: Tenacity and Mercury's Treds

I honestly just found out about this recently and i was kinda dumbfounded. I thought it was a bug when i only had 51% Tenacity instead of 60%, but then i searched around and found out that the rune and stacks multiplicative instead of additive. I have no idea why its this way though, since the rune is already rarely picked and this only makes it feel like im wasting stats and not getting the full value that i could have gotten out of the other runes. Since you already get 30% out of the rune, adding on top of that makes you feel like you are losing out on stats from the itself.