My guess is that open offices make the company seem more cool/laid-back and less stodgy than cubicle farms. This might help to attract employees, even though it actually makes them less happy in the long-run.

This is it, basically. You see it a lot in companies based on churning through employees rather than building up a stable longterm workforce. The open-plan spaces look hip and make newcomers feel like they're working in a Cool Modern Company, so they're more willing to endure the daily annoyances like half a dozen distracting conversations going on at once across the room. It doesn't matter that they eventually wear down under the realization that they are working in a Panopticon prison yard. In fact it's probably considered a feature instead of a bug - I can't think of a better way to make employees feel small and pressured to perform.

Cubicle farms might seem like the prime example of drudgery, but at least you get your own little space and have an unexposed back.