It's not. People want to create a narrative that there is a tension between them, one or the other. (Urbanists hate cars because they hate cars, so they want to define every argument as requiring a hatred of cars to make any progress on anything else, to suit their religious beliefs. Every blog post on StrongTowns is a good example of this).
But you can always do both well, we just choose not to.
Chicago Loop, while not perfect or ideal, is a great example of how an urban environment could look that serves all well. It is dense, highly walkable, highly transit oriented. But it also still has a strong network of freeways and has an entire underground network of roads and tunnels for cars to pour into/out of (where they can peacefully exist just a few feet above/below walkability and transit, without ever hurting any of it). As those cars electrify, all noise and pollution problems are slowly but steadily disappearing.
In many ways, it is the closest to an American city ideal. Which I would define as a place where it is 100% convenient to use any transportation method you wish, at any time you wish, with as few-to-no restrictions as possible.