Every startup I've known that's attempting RTO is definitely doing so from VC pressure. Of course the announcement from the leadership will never phrase it that way, but when leadership suddenly has some inspiration about the value of working in proximity immediately after a board/investor meeting you can be sure where that inspiration comes from.
Most recently IPO'd startups are also strongly influenced by small numbers of institutional investors that have major sway in decision making. All of those investors have broad portfolios and have a keen interest in keeping commercial real-estate healthy.
At a certain level that pressure comes from the CEOs themselves since they likely have a large number of assets (and/or the company itself does) tied up in commercial real estate. All of those mega-campuses are real estate investments and have a certain value on the books. If the commercial real estate market crashes, the value of those assets does as well.