I think we're talking the difference between $300/month on open desk space vs $1000/month on an office. It's a lot of money and it adds up, but these people are costing the company a total of $10k-$40k a month
My theory is that open space is extremely helpful to certain types of people, and those traits coincidentally overlap with the people calling the shots at a lot of companies. Sociable upper managerial-types probably value being able to collaborate more easily in open space more than being able to concentrate in private offices.
Plus it's harder to get away with blatantly goofing off if anyone can look over and see your screen at any time, but I dunno how big a deal that is when people are setting their own hours and picking the projects they're working on. At least in my case, I don't give a crap if my teammates have Facebook and Netflix onscreen at all times so long as they're getting their jobs done