I kind of wonder how much open office really is about cost. I mean, it definitely does matter for startups, but I don't think that's a factor at FAANG. I mean, look at this new FB office, https://www.youtube.com/watch?v=FmTZJRLtA30, there is a ton of open space, they could probably sacrifice just a bit of that for individual offices, or even keep it how it is and add offices.

Because probably only larger companies (very difficult for startups to innovate here!) can actually afford individual offices, and it's not exactly trivial to re-arrange, I think it just takes a long time for the conventional wisdom to change.