If only it were that easy! If offices could be easily converted to residential, then the current market conditions would be incentivizing owners of all these vacant office spaces to be pressuring for re-zoning and conversion right now. Unfortunately, many office spaces are ill-suited for residential conversion.

There's often not enough plumbing. Often very literally, as in the main sewer, and municipal infrastructure, are undersized for residential use. Also, once you add adequate walls and dividers to separate the units, plus furniture, packed into typical apartment sizes, the weight is usually much more than for an office, often more than the structure is rated for.

An office only needs one central water/sewer connection for the bathroom and kitchen. Renovating concrete floors to add additional water/sewer connections - and renovating the main water intake and sewer lines to handle the additional volume makes office -> housing renovations more expensive than flattening the building and starting over from scratch (which is obviously horrible for the environment)

or maybe some type open community work/art space? you'd have to do it in a way that doesn't distract the employees who do choose to work in office though.