Why is it that manaless champions tend to have high sustain?

Mana requires you to manage your spell use carefully, because running out of the stuff makes you a lot less effective. A champion not having to worry about mana at all is already at a massive advantage. They can't be forced to back unless they lose too much health to continue, unlike mana champs, who have to back when OoM. So why is it then that these manaless champions have such ridiculous sustain?

All have absurd built-in sustain and have no mana to manage.

Have no built in heals, but synergize incredibly well with sustain items, such as Hextech Gunblade on Kat, Ravenous Hydra on Kled and Riven, and lifesteal in general on Shyvana and Yasuo.

Are manaless with no easy access to sustain from items.

Seems that energy based champs are lesser offenders than the others, which is odd considering they at least have SOMETHING to manage.

So why is it, Riot, that champions who can only be forced back by nearly killing them tend to have incredible sustain to make that almost impossible?

EDIT: Forgot Gnar existed, added him to the bottom list