the reason you see mages is because by design they have more damage early than most ADCs, so they can bully the lane and get kills
also the reason why you see very specific mages and not "all" are because generally speaking support is the role now where champs that can't cut it solo lane go. So you look at Zyra and Brand (Btw Brand is the most volatile champ in the game, so he either feeds or pops off) and see they can't really play mid with the amount of all in jump every where Damage, but you put them bot and they do pretty well because generally speaking there won't be any one in the lane early who can match their dmg.
I use to be a support main, honestly even as a ADC i'm still fine with mages there. Zyra is a bigger problem then Brand tbh but they both have strong damage for sure.
I mean the champs should be playable and it's clear mid is just way too much for their immobile kits so i mean it's one of those things where it's like Where else can they even be played? I guess you could lock them in as the ADC which would be fine since their kits are more similar to a immobile adc's design.