Because working is a collaborative activity, not a social activity.

By nature, social activities need your brain to be focused on things happening around you.

Work activities need your brain to focus on that task at hand, and not on your surroundings. Hence your brain needs to overcome all the distraction to focus on work, if you're in an open (social) environment. No wonder people feel exhausted at the end of the day.

That said, I think having a completely closed office is equally bad. I think a good office is one that combines the best aspect of both.