... so why are mages the only class in the game who are expected to build it? Aside from a few fringe cases, the only champions in the game who have mana built into to their requirements are mages, and they're the only ones who are significantly gated by it.
#Logic?
Slythion5 votes
+ 
are gated by it in laning phase
and 
are gated by it with their early clear options
uses mana as a method to punish bad axe management
mana gates his early game and build path
is definitely gated by mana
Most tanks are gated by mana during lane as well.
The assumption that mana only has one purpose and one use is too one-dimensional, and the idea that only mages are affected by it is easily disproven in my opinion. The difference between mages and most other champions that use mana is that mages continually are gated by mana and are designed around their ability to use it, while other champions or classes are simply gated by it to not be too oppressive until options or door become open as they scale into the game.
Cocho4 votesother classes dont throw their kits at the wave to clear them (as much)
ZephyrDrake2 votesmages have something that other classes have (with some exceptions here and there)… waveclear. Every champion that has anything above "not completely terrible" waveclear is gated in their mana in some way or capacity
Meep Man2 votesBecause the rest of the classes don't spam spells as much as Mages or Supports. Supports still build mana regeneration as well. Not many other classes have their main CD spells on a CD lower than 5 seconds and those that do have those spells having minimal impact or requiring to be in melee range to do it.
Who Fed Ru1 votesAre you really just saying it doesn't make sense that mages are gated by mana?
Teridax683 votesI think the issue is more that mages are the only class who are essentially required to build mana. Out of the five remaining classes in the game, four generally don't need mana at all, and the one that remains, i.e. supports, have mana regen in their itemization instead, which renders the user pseudo-manaless after enough items. We are now at a time where the members of the most mana-centric class in the game end up with the lowest base mana pools, which in spite of the intentions behind that (i.e. itemization), feels counter-intuitive.
LilBadWitch4 votesHe clearly said:
why are mages the only class in the game who are expected to build it
He's implying that other classes / champions should feel the need to build: 
That they're not mana-gated as heavily as mages are.
Koechophe3 votesNo, I'm saying it doesn't make sense that no other class is.
Who Fed Ru1 votesit actually makes sense, quite a lot if you ask me
ZackTheWaffleMan3 votesNo, he's saying the exact fucking opposite of what you just said.
AlienPrimate1 votesWhenever a different type of champion has high mana costs, they end up having terribly low play rate and get reworked. Exampes are Galio, Poppy, Yorick, Urgot, Taric, and Sion.
Automated Riven1 votesAll of their Mana costs were used to game problematic gameplay. Not just damage.