Because otherwise the social safety net--Medicare and Social Security included--will collapse.

Maybe we shouldn't have those programs structured as they are now. But we do, and a shock to the system will result in unprecedented economic misery and social instability. If we want to talk about how to deal with demographic change, there's an important long-term conversation to be had about preserving the social safety net. But any solution that wants to avoid a potentially-catastrophic economic crisis must have a serious, realistic answer for this. Not building housing is a fundamentally unserious policy, driven by a preference for the suburban lifestyle and the fear of change more than any realistic concerns about sustainability.