The simple reason being that the biggest IT/tech companies have invested heavily in land & buildings (think about how Apple and Facebook have built huge "campuses" - why they call the offices/building a campus, I don't know, but anyways ...) and they need to utilize those assets in order to avoid the "white elephant" problem.
Also, unlike "agile" software-development or on-demand hosting, they can't merely downscale their land & buildings as they would for their AWS clusters and they also can't sell these buildings, as they seem geared specifically towards the needs of their owners (it isn't just a regular old office-block).
So while some startups of the future will embrace a remote-first culture, the big tech companies and many other regular startups will continue along with the concept of a fixed-location and fixed-office well into the future.