I've worked for several big companies (though not Facebook, Google, Apple, or others in that class). None of them showed any interest in analyzing developer productivity, much less looking for ways to increase it. My gut instinct is that developer productivity is hard to measure, real estate costs are easy to measure, so you end up with 500 people in a noisy environment doing the work that 100 could do in a quiet environment.

I also suspect there's a status thing going on. People working on the floor are lower status than people with offices, and I suspect some execs find status signaling more desirable than productivity.