1. There are, in fact, tech companies that still demand employees come into a physical office. There comes a tipping point in a laborer's market (tech being one of them) where this becomes sufficient reason to leave when there are plenty of reasonable alternatives that don't require employees be physically present.
2. Mental health is often overlooked in these discussions. The pandemic has been a huge source of stress, uncertainty, and general chaos. Many people (myself included) lost their usual outlets of stress (going out, meeting with friends, catching up with family, etc). From personal experience, this lead me to having a spat of time where I was burned out and had to lay off working at all for several months before joining a new organization. From what I hear, I'm far from the only one who's gone through such an experience.
3. In light of COVID, many people are facing the realities of mortality much earlier and more frequently than they would prior to a pandemic. Many of us have lost friends or loved ones if not to COVID, than to COVID caused problems (mental health, substance abuse, health problems that became critical due to lack of ER capacity). Having a brush with death is a strong incentive for people to reevaluate their situations and reexamine how they spend their time. For many people, work is not fulfilling and they may be more willing to adjust their lifestyle to prioritize things important to them that don't require as much money; or things that take them to other careers; OR give folks a kick in the pants to demand more from their current employers.
I suspect there's more to this trend; more nuance than is being captured by the current news cycles. The US has a diverse population of people in a variety of situations and the driving factors for hiring trends for bay area companies are likely not indicative of what employees are seeing from their side of the table.