I think when a lot of people say high density, they don't mean 100floor residential buildings are all the housing, but tend to think of something more akin to densely packed midrise buildings and green spaces. If you have the later, there simply is not space for cars and high density. Large universities come to mind, where there may be 50k people using 1-2sq mile while 5+ story buildings are rare. You would have to walk a mile or two to get to a car to drive 6 miles around to the other side of campus at 5mph to walk a mile or two to get to you class 1000ft away from your starting point if they were car centric.

You don't even need public transit at that level of density but it's an option.