Because actions have consequences and action at a scale sets norms. Considering he has the choice to walk away and is exercising it, it is sending the signal to the market that forcing people to return to office will have consequences. If he doesn't and everyone doesn't, it just kills the work-from-home momentum at large.
In fact I am grateful to him for doing so because I am not in a position to do so myself but would certainly like to reap the benefits of such actions.
You will also likely reap not only the benefits of such actions, but also the consequences, because fully remote companies have little reason to choose employees living in a high cost places like Silicon Valley over remote developers from far cheaper places who will agree to work for 1/4 of the compensation paid to office workers.
In fact I am grateful to him for doing so because I am not in a position to do so myself but would certainly like to reap the benefits of such actions.