Why isn't this done to handle heavy car traffic in densely packed cities?
(As an aside, in earthquake country, modern large expensive buildings actually rest on some kind of roller/springs/etc with dampers, so the building can move rather than break. [0])
Before the modern era, flying buttresses [1] were needed to support the then-largest buildings (churches etc). These days modern steel with rebar concrete and other techniques make those unnecessary, but I wouldn't be surprised if they would be needed even now to act as pillars for bridges, elevated freeways, etc.
I may be misunderstanding something there, but it's not clear that you're going to get an answer from a skyscraper civil engineer today.
probably would also complicate ownership & liability