[ghaff]

It can be both. Prior to the current situation, many/most companies preferred people who could/would commute to a company office. Which gives more options to people willing to work near one of those hubs and mostly work in an office.

At the same time, many people prefer to live near one of those hubs whether because they just like NYC, Boston, Austin, Bay Area, Seattle, etc. or because they believe it gives them more flexibility in changing employers. (And/or being in proximity to many like-minded individuals.)