No no no, you see, if the employee isn't there, they could rent out that space instead, but they don't. By getting the employee back into the office, they're eliminating those opportunity costs! /s

On a more serious note, saving costs could be a reasonable argument if the company were compensating the employee for their increased cost of living when working from home - electricity, heating, water, internet etc. at home also have to be paid somehow. However, I kind of doubt that a significant number of the companies we're talking about here actually does that in the first place.