Good point. I think a couple things are possible here:

1. They can't outright tell employees "we just realized you were getting a better deal than we bargained for, so now we are re-negotiating." Kind of like they couldn't tell us "you had too much of a good run during Covid and ZIRP, so now sit down," and instead did the layoffs. So they have to couch it in other terms like collaboration and watercoolers. I suspect most RTO policies will have some flexible language to the effect of "existing remote employees can figure it out with their managers," and part of that would be a renegotiation.

This is not without precedence: a similar thing that happened during the height of post-Covid remote work was that employees moved to very low-cost locations, so most BigTech (and other) companies then instituted pay bands that depended on your location. To be fair, they were still very generous salaries for the locations in question, but the reasoning they used was "our pay is competitive relative to the market, and your market just changed." Those must have been some fun conversations for managers but they happened and most employees accepted.

BTW a common argument then was "how does my location matter as long as my output provides the same value." Employers interpreted that as "Hmm, location doesn't really matter, so why not pay somebody across the world who can provide the same value at a fraction of the cost."

2. They are not interested in direct renegotiations and the RTO wave is still part of a "silent" layoff, which also has the side-effect of depressing salaries. Those who remain will be less likely to ask for raises, or be more willing to renegotiate to get back their remote perks. Those looking for new jobs will naturally enter a negotiation phase if they get an offer.

There may be other mechanisms at play too.