Insomnia and Altissia are from XV.
Think about Academia like this: the concept is a sprawling megatropolis that likely kept building up and up because of lack of room to expand outward. That's fine as a concept. But the limitations of development and hardware does not emphasize the sort of overcrowding you would expect. There aren't a bunch of people walking up stairs while people of privilege are able to take the quick and convenient escalators. The neon signs are there, but it doesn't feel super gaudy as if it's a desperate attempt for store owners to bring people into their shop as opposed to the dozens of similar shops. It doesn't even seem that different than some places like in Tokyo. The buildings are all pristine and a white/light grey tone, with no personality. In a city so crowded and with such tight spaces surely it would be difficult to maintain cleanliness. I could go on and on with examples like this.
The point is the city seems very superficial, and that it relies on common tropes to convey the futuristic setting amplifies the artificiality. So to do the countless walkways and escalators. It doesn't feel like a cohesive part of a real city, but rather a basic sci-fi city that Square Enix didn't have the time, resources or desire to make into an actual open world or semi-open space, so they crafted a bunch of corridors and smaller areas almost randomly connected and put a fairly basic backdrop to make it feel bigger and more open than it is.
By the way Altissia is like a pseudoscience, Final Fantasy version of Venice, Italy. It does that tightly constrained city better and which a truly awesome design and aesthetic. So I'm not disparaging Academia just because it has a lot of corridors.
