I would agree that it's the main reason, but I'm not sure it's always the only reason. Some people (for whatever reason) actually buy into the idea that working in an open space is better for creativity, collaboration, etc. Even some developers, who ought to know better, seem to have drank enough Agile Koolaid that they believe in this stuff now.
It's actually, IMO, one very unfortunate side-effect of the Agile movement... people somehow started equating "agile" with "everybody must be in one big room together" and that meme stuck, and now it's contributing to the propagation of this "open plan" nonsense. Never mind that you pretty much never, ever, need to actively collaborate with somebody for 8-10 continuous hours in a day. Or never mind that Tom and Suzy collaborating is keeping Joe from getting anything done due to the constant distractions... sigh
They're called extroverts. They get into management and then fuck everything up for the mass of introvert engineers because they're incapable of conceiving that a human being wouldn't want to be in constant contact with other human beings at all times.
I think many of the open plan office advocates have no idea how much they produce on a given day or not, and don't simply know how much MORE they make in a quiet room.