"X should not have Y" is a flawed way of thinking by itself.
Manaless champions are designed to not be gated by mana. What other thing they are gated by is not that obvious.
Yasuo is gated by the need to be close to enemy, without a reliable way to disengage if the fight isn't going well
Katarina is required to go in to do her job, with her combo being much less reliable than other assassins' combos. She has resets, but they are effectively useless in 1v1, which is how assassins usually prefer to fight early-mid game
Garen can't "spam" his manaless abilities because his Q is his only gapclose/escape tool, his W has a huge cd and his E is melee
Also, I only found 2 manaless champions with resets: Katarina and Tryndamere, Again, Kata can't really "spam" her abilities and her resets only works in group skirmishes and teamfights while requiring her to kill someone first. And Tryndamere's "reset" is a cd decrease on his E whenever he autoattacks, its less of a "reset" and more of a "if he uses his E to escape, it has longer cd"