1) Cheaper -- different budgets. Usually the people specing the office are judged on how cheaply they can deliver (partially), not on anything related to employee productivity.
2) Open offices look way better in photos/concepts -- so they'll get chosen over a boring all-offices layout
3) Much easier to reconfigure -- this is why a lot of startups or those in high-growth periods end up in them, even if they'd prefer offices. Even cubicles require some professional reconfiguring, but anything requiring permitted construction is a big deal.
2) Open offices look way better in photos/concepts -- so they'll get chosen over a boring all-offices layout
3) Much easier to reconfigure -- this is why a lot of startups or those in high-growth periods end up in them, even if they'd prefer offices. Even cubicles require some professional reconfiguring, but anything requiring permitted construction is a big deal.