Executives at pretty much every organization offer the same sensible reason. Working in an office together greatly reduces the cost of collaboration, which is valuable in its own right as well as leading to more inventiveness and better mentorship.

The problem is that, as you can see a bit upthread, a lot of ICs don't notice or don't care about the collaborative aspects of their job. To someone who feels that writing out lots of code is their real job, that sensible reason sounds like a weird deflection, since "reduces the cost of collaboration" means by definition that I'm getting interrupted or distracted when I could be heads-down programming.