The thing is, even in the densest cities in the world it's not like people are completely without cars. They just manage to get by with one car per family rather than one car per driving age member of the household.
It has the additional advantage of letting their teenaged kids get around and do shit on their own without having their parents play chauffeur all day.
Between car sharing services, bike shares, and on-demand ride-hailing it's easier than ever to get around without a car and fill in the gaps that a transit system leaves. The only thing that actually gets hard is being able to drive out to go camping or manage the extemporaneous pick-up of free-shit on CraigsList. Even that would be doable if car rental companies were more focused on long term rentals for road-trippers rather than just people who need commuter econoboxes to leave their hotels.