From my (very limited) understanding, the underlying reason is the health of the national economy.

A bunch of businesses giving up their office space would destroy the commercial real estate market, and that could trigger another economic recession/depression. It could take the economy years to recover, costing companies billions, and bankrupting some of them. Even fierce competitors will work together in order to prevent that from happening. (I’m not sure how realistic that fear is; I’m just explaining their reasoning.)

So, while an individual corporation would benefit in the short term by moving to a building that’s only one third as big, the long term risks to the economy scare them off. In fact, the only reason working from home is still being discussed is because there’s a shortage of skilled workers. Companies can’t dictate terms quite as strongly as they could a few years ago. Employees see working from home as a major perk, or even a necessity. Inflexible companies invariably lose some of their best people, so they have to allow at least a few work from home options.

Personally, I like seeing corporations being forced to compromise.