What are the psychological or social mechanisms behind this?
I'm rather of the kind who don't like to be told what I should do so I can't relate much. (I'd appreciate it if people would tell me they would be pleased to see me and that could actually make me come, but that's different than some emotionless guideline)
As one example, consider the "unlimited vacation" policy and pushback around those. Sure, the official policy is unlimited, but try taking more vacation than your manager wishes you would and see how that affects your next performance review.
Okay. I've not experienced this situation indeed. Official policy / stated expectations has always matched actual expectations perfectly well wherever I worked, to the best of my knowledge. A difference there must be a mess to handle indeed.
You might think that the people who get to Google are independent thinkers, but it's quite the opposite. Look at it this way: they spend 12 years in primary and secondary education being told what to do, and they study standardized tests to advance to the next stage of their education at a university.
They spend 4+ years there, all the while taking tests and exams and going through courses where again they are told exactly what to do every step of the way, with granular feedback as to how they are meeting the standards. Then upon graduating with a degree they put their knowledge to use by studying for what is essentially yet another entrance exam into Google.
The person who follows this kind of trajectory hasn't had an opportunity for truly independent thought probably their entire life. They are conditioned to follow guidelines, and the first questions they'll ask for any task are: "Where is the rubric? What are the rules? How will I be evaluated?" because they know how to succeed when there are guidelines and rules and procedures to follow. Take away those, and they will flail. I've seen it many times.
For the people who prefer to be in office a few days per week, it would be nice if those few days also overlaps with the in-office days for the people they wanted to meet in person, and it's easier to make this kind of coordination at a higher level. Thus management guidance is basically "let's pick these day(s) of the week where our team would try to be in the office together".