I disagree that property value is the main driving force behind return to office mandates, but if they do in fact own their offices, then those offices depreciate in value. They also cost money in upkeep. Gardeners, janitors, window cleaners, security, power, maintenance, insurance. Even if nothing is happening, there are contracts with companies that provide these services. Either they'd have an agreed frequency of visits, or a retainer fee. They have managers that want their employees to make on site visits to justify their jobs. The AC and lights would need to be on for anyone onsite, including security, and that is a lot of power. Offices are expensive.

If nothing is getting done in those offices, then those offices are bleeding money. And if they're empty and nobody wants to work (in an office) anymore, then you can't sell them for anything like their old book value. It's a huge loss.

That said, I suspect it's more institutional inertia that drives most of this return to office stuff. Managers justifying their existence or just not being able to cope with the new skills they need. Plus selling or ending leases is a job that someone has to do, and you might fuck that up, so you'd rather not make the change. Plus they've got the offices now, what if they need them soon? Capitalist companies, especially corporations, are fundamentally conservative entities.