It's not just the corporations renting those offices, it's the politicians of downtown areas that fear a downturn in tax revenue due to more empty offices and less people getting their daily coffee/lunch/after work drinks.
And of course, if everyone's working remotely, this means it's a lot easier to find a better job without even needing to leave the house to interview, which gives employees a better bargaining position (downside is that employers will start looking at employees in lower paying countries as well).