Resource Mismatch?

I understand it from a balancing point but why do strictly auto based champions use mana? Using Fiora as an example, all of her skills are skill based off her lore. I feel like this is a missed opportunity for unique resources or interactions. Like hitting Vitals fill up her technique bar for a certain champion, making her more powerful against them.

This could be argued for any champion that doesn't use "skills." Most champions if this type already use this concept like Tryndamere, Yasuo, or Renekton. Decent mana user example may be: Master Yi, Graves?, Xin Zhao, Fiora.

Not a rant, just a curious post. What resource do you think makes 0 sense on a champion?