Because then transit, services, restaurants, stores, dry-cleaners, gyms fail, and the taxbase collapses (every city has a different mix of commercial vs residential property tax vs sales-tax).

DowntownRecovery.com project mapped this (using cellphone user data, at least)

https://downtownrecovery.com/

Prior discussion on HN: https://hn.algolia.com/?q=downtownrecovery

Prior discussions: https://hn.algolia.com/?q=downtownrecovery