The city was very clearly built from the bottom up with many buildings Frankensteined together to just look right from the street. Most roofs have nothing on them (although there are some exceptions). Many buildings have unfinished roof collision geometry that only very roughly trace the shape of the building and walls and structures on them that have no collision at all. A couple points of interest are clearly only intended to be accessed from below and not exited from above, with death planes surrounding them.
There's also at least one instance where they forgot to lock the upper access to a building (in this case breakable windows), so you can get into areas you aren't intended to be in early. These sorts of things would be much easier to find with flight.
I wouldn't mind a flying taxi service or something if you just want to take in the sights, but actually being able to fly would greatly harm immersion and provide no tangible benefit to the player beyond easier travel (which probably wouldn't better than Kerenzikov to be honest).
If one of the expansions is a redesigned upper half to Night City with building geometry finished, Frankensteined buildings given custom collision points so they actually look right from above too, activities on the upper levels of buildings to participate in, and something put in place to keep us out of where they don't want us that's better than invisible walls, I could see the value in airships, but that's a lot of groundwork to do first. Admittedly I would hope some of that work gets done anyway in the name of completing the game, but I digress.