The value of the building is as a multiplier of the net operating income, that's what a cap rate is. If people come back in and companies expand and renew their leases, that increases the value of the building and reduces the capital needed to recapitalize the mortgage.

And, bringing people back in, especially in the context of e.g. Amazon with thousand of workers, also has lots of knock on effects on other businesses, and there are plenty of copycats who follow the big seven tech companies.