Because had it been any other way, the dialogue wouldn't feel as natural and realistic and "muh immersion" as it is now!
But it looks like in Bethesda's attempt in making Charisma an actual meaningful attribute by having it affect dialogue, they forgot everything else that could have an effect on it as well. Or it is just another casualty of the voice acting four button dialogue system. Who knows why they do what they do anymore.
I remember when specific perks and attributes had their own special dialogue to them that rewarded you for taking those options for your character. Moira asking me to go to the RobCo. Facility when on the contrary, my character was a Robotics Expert. Why, I could tell you all about robots right now and save the journey. Those moments felt good. And are quite missed here.