People think open offices are some sort of hidden conspiracy that's been set up to make their lives hell. There's no conspiracy; open office floorpans require less floor space per employee and are thus cheaper. Think about all the additional space required to have walls, a hallway connecting offices, etc.

Firms are not stupid. I am not doubting that there is a productivity hit due to distracting noise and motion, but it's probably not outweighed by the reduction in real estate cost.