Open offices photograph better, have been on the right side of the modern&cool™ narrative, are more flexible to rearrange, a bit cheaper to rent and maintain and there's always a sense of busyness going on.

It's very easy for managers to keep track of their herd, see which face they don't like and flex to the colleague/competition using the always important "how many workers I have under my wing/foot" metric.

I think the only downside of open offices is that productivity takes a significant hit and makes worker health and morale a nightmare. But other than that they are fine.