You join a union, or you join a different company. Or you threaten to, at any rate.
Understand your leverage: job marketplace bargaining power stems from your willingness and ability to do the latter, while structural bargaining power stems from your willingness and ability to strike and/or engage in sabotage.
The film industry unions famously pushed to centralize all of the industry into a 30mi radius in LA (TMZ- thirty mile zone) [1], so it's totally possible to push for location-specific policies.
The Alphabet Workers Union has been fighting against RTO mandates for a while now, AFAIK. Not super successfully, but they've gotten some extensions and delays a few times.
Would a SWE union push for work from home? I don't know how software unions have fared in the wild so far, or even any other type of engineers union in terms of work-from-home.