I'd speculate some combination of control over employees (poor management practices, etc) and making use of owned land/offices that are difficult to sell otherwise. Not much else makes sense to me, especially for tech companies where nearly the entire job exists in virtual space of some kind - no wrenches to turn.

Edit: Someone else suggested a way to "lay off" folks by having them voluntarily leave the job to avoid the return to office. That also sounds pretty plausible to me with the extent to which companies are starting to squeeze with what feels like an incoming recession period.