Demacians have a thing against mages.
With the new reworked Galio and The story the Soldier and the Hag. I'm pretty sure that mages are hated fiercely in Demacia. Just think about it, their city was built from running away from mages. Jericho swain, the leader of their sworn enemies is a mage on top of that the amount of mages that noxians can produce seems to be a bit absurd
(yeah i know gameplay wise LB's an assassin but like she's still magic.). Demacians such as
dedicate their life to the destruction of magics. When Garen finds out his sister is a mage, he treats it as if his sister were gay back in like the 1940's he's super torn up about it and feels a bunch of mixed feelings about it. This is further supported by the fact that Lux is the only real mage we know of that's 100% from Demacia. Apparently she's also powerful enough to make Galio come to life. Which means that Lux is the first mage from Demacia that's actually really powerful for a good long time. He only ever came to life when enemy mages showed up so its been hundreds of years since Demacia had a mage fighting for them.