Because Kat was like a younger sister who was protective of her family, and only resented the presence of Noble 6 because of the reason that spot needed to be filled: because she had failed.
She wasn't a girlboss. She was family, united with her brothers in arms with shared pain and overwhelming loss. That she died in our arms from a cheap shot by a Sangheili sniper made her death all the more tragic, and made it feel all the more real.
They don't make stories like this anymore.
I hope they will again someday.