It's probably cheaper and certainly more environmentally friendly to keep building upward in urban centers than to build mass transit to exurban areas. Urban areas already provide significant subsidies to exurban ones via taxes and cross-subsidies - see, e.g., [0] and [1]. The comparatively high cost of living in high-density areas is driven by a combination of those transfers and pricing distortions created by local governments, not by the fact that high-density housing is inherently less cost-efficient (and I'd argue that the opposite is true).
It's certainly not cheaper, for developers, to build highrise aparments in the city than upscale tract homes on tiny lots in an exurb. And environmental impacts and the responsibility for transportation are both externalities easily pushed to someone else. This is what actually happens.
>It's certainly not cheaper, for developers, to build highrise aparments in the city than upscale tract homes on tiny lots in an exurb.
Why's that? At the margin, at least, it seems unlikely that this is the case - it's probably quite a bit cheaper to add an additional floor to a 50 unit/floor high-rise (when it's being constructed, not after the fact) than to build 50 detached single-family homes in an exurb, for example.
It's true that externalities can be, well, externalized, but that doesn't mean the difference in absolute costs shouldn't be considered. At the end of the day, someone still has to foot the bill.
[0] https://medium.com/by-the-bay/financing-suburbia-6076dae990f...
[1] https://johnhcochrane.blogspot.com/2018/06/cross-subsidies.h...