Yes, I am a NPC

Sure, the Northern Tribe might have benders, but they have such a different culture/identity around bending compared to the South. We see in Hama's flashbacks that the Southern Tribe had male and female benders at the helm, fighting equally, learning the same things, sharing the same tactics. They weren't left behind as strictly healers or defenseless women given a man's gift, they were strong.

And the thing that separates Katara and Aang's stories being the last benders of their nations is that Aang remembers what it means to be an Air Nomad. There are obviously blind spots because of how young he was, but Aang knows the customs and the culture and spiritual practices. He was able to pass those down to Tenzin and his grandchildren, he was able to start the process of rebuilding what was lost.

But Katara? She was even younger than Aang was when the Southern waterbenders were eradicated. There are Southern practices that are most definetly just... a fractured part of history because she never got to experience them. There are ways of hunting, of building, of navigating the waters, ways of healing, probably even spiritbending, that are simply lost to time.

And maybe her grandmother can clue her into it, memories from a nonbender living aside benders, but it's not the same. She didn't get to hear it from the waterbenders, she didn't get to learn from a Southern waterbender, she doesn't even know what to rebuild because it was never something she was versed in.

That's why The Puppet Master is such a heartbreaking episode. Because Hama could have been the Southern teacher Katara clearly wanted, she could have shown her more, but ended up passing on her legacy of Bloodbending. And on the inverse, I can't even fully call Hama a villain for wanting Katara to know how to bloodbend, because the ache of thinking you were the only one left for so long, the rage against a Nation known for destroying nations, would've driven anyone insane.