Because closed offices are expensive, limit your options, and are less flexible. A company with 10 people now can scale up to 15 or even 20 in the same space with an open plan, but would have to move offices and break leases with a closed plan.

Another way to say the some of same thing is, for the same office budget, you can get a nicer place in open-plan format than closed-plan. When looking at offices, people tend to prefer the nicer offices and nicer locations. Hence: open plan.