Many reasons but the two that seem to be the most common are that managers don't feel comfortable evaluating employees on output rather than time their butt is

Many reasons but the two that seem to be the most common are that managers don't feel comfortable evaluating employees on output rather than time their butt is in a chair and many people (ICs and management alike) use the office as an outlet for their social needs. Perhaps the third reason, though less common, is the belief that the best ideas come from hanging out at the water cooler.