I should clarify myself, I bought couple shifter cables (individual, non-specific) from my LBS and all of them are measured the same length as about 80 inches long. I thought 80" must be long enough for the rear but I can't get it long enough to reach to the RD. I know there is no loop or anything somewhere as I pulled the cable couple times to make sure. I am using the same cable housing so it can't be that. I am confused

That's how I routed my cable. Cable goes into the right shifter, along the handlebar and then goes down to the cable guide which is on the bottom of the BB shell. Straight into the cable housing on chain stay, loop around and goes into the RD.