Because it is.
I mean, I think there's a pretty clear difference between
- HYPER HYPER magical setting, where nothing more than vaguely like IRL urban environments exist, and
- HYPER HYPER modern-urban setting, where nothing more than vaguely like magical things or events exist
I would not classify Ravnica as "urban fantasy", even though it is literally a city-plane. I would classify it as high magic in an urban context/location. It's sort of like the difference between "Romantasy" and "Romantic Fantasy". The latter is a subgenre of fantasy where romance elements
are included, but they are NOT the primary focus; the romance elements are distinctly secondary. The former is a subgenre of romance where fantasy elements
are included, but they are NOT the primary focus; the fantasy elements are distinctly secondary.
Some might see this as "you got your chocolate in my peanut butter" or the like, but it really is a distinction that regular, everyday folks have shown they care about. It's why you get responses to books where one person is bitterly disappointed and the other is elated, and the latter says to the former...
"But I thought you said you LIKE fantasy books."
"I do, I just WANTED a romance!"
"It is a romance!"
"No, it's NOT a romance, it's a fantasy story that has a romantic subplot!"
"They're the same thing!"
"NO THEY ARE NOT."
In my personal, likely-uninformed opinion, Ravnica is a High Fantasy setting with an urban focus.
Shadowrun has a (dystopian) urban-fantasy setting: it is very very much the urban environment we're used to, plus or minus some extra dystopian grunge and the like, but it also has magic and orks and spirits. That is what most self-professed "fan of urban fantasy" folks would want, other than the dystopian element (many do want that, but not all). They would not want "Lord of the Rings, except it's a fantasy megacity"...by and large. I'm absolutely certain SOME people who call themselves "fans of urban fantasy" would want "Lord of the Rings, except it's a fantasy megacity"; I'm even fairly sure that you can find at least one other person who would think it bizarre that someone might consider Ravnica different from what "urban fantasy" means. Doesn't mean that that's what the typical person is going to understand when you say, "Ravnica is an urban fantasy setting."