It's a combination of corporations wanting to avoid depreciating the offices they own/feeling like they're wasting offices they're leasing (sunk cost fallacy, ho!), management not wanting to lose out on the cushy offices they've been given as rewards for their promotions, and management that wants to cling to the narrative that there's actually any point to the micromanaging busywork that comprises their entire jobs. There are also concerns about the ability to motivate people with non-cash incentives (which do actually tend to motivate better than simple raises, but only because you can see all the people who didn't get that incentive to remind you of what you achieved, whereas a new salary just becomes the status quo), which means corporations have to spend more money handing out raises if they want to motivate people.
Of course, all of those are corporate problems, not anything to do with productivity or worker welfare. There's also a baffling lack of interest in the most obvious solution to the empty buildings problem: Rezone them as residential and convert the buildings into housing. There are massive housing crises in every major city in North America. Turning downtown office buildings into downtown condos would be massively lucrative for their owners (who preferred to develop them as commercial because it yielded a better return per square foot, but now they're getting those same returns without needing any office space) while also facilitating higher-density city design and increasing housing supply much more cheaply than doing new builds.
I've outright told my manager that if showing up in the office purely for the sake of showing up in the office is a required work activity, I'm not leaving my house before I'd normally start work. If it's a work activity, either I get paid overtime to do it outside of work hours, or it doesn't happen outside of work hours. Sure, that means ~40 minutes less actual work those days, but that's not my problem. I'm also perfectly content to be present for my actual work hours when my work legitimately needs an in-person presence, since then commuting isn't a work activity.