At my FAANG they did studies and on average people are working MORE hours each day since the switch to WFH. If they were concerned about maximizing hours then they'd leave it as in. I suspect there are other concerns.
This had been my experience too. I don't think I had been more productive - many times I've just stayed late poking at the problem I'd normally leave for the next day when working in the office. Most of these times I still solved it the next day with the new ideas after a night's sleep. Not to mention I've got many ideas from random discussions with colleagues, which ceased with the WFH. So return to the office is a win-win for me and my employer: I work less and I am more productive.
I have to wonder whether they're available for more hours or actually putting in more work. Home distractions tend to extend the work day for people who have pride in their craft.
At least for me, the hours I used to spend driving my car are now spent doing actual work, because that’s what I’d rather be doing anyway. The only reason I don’t work more hours as it is is that I have other obligations. Just seems like a total waste of resources along multiple dimensions to have me driving around my brain.
There could definitely be other concerns too. Such as security. It'd be much easier for someone to copy a code base, or get evidence of wrongdoing by your employer, or whatever, when at home rather than in an office. It's also harder to secure all your employees' home networks than one office network. But I wonder how many people who work at offices still bring their laptops home to use.