Personally, I think it's because people find it easier and less taxing to work in a group than alone. When the not-so-rare exception occurs, the differenc frightens people.
HabitableZone — be civil, or at least entertaining.
Masthead: Earth photographed from the International Space Station (NASA);
the spire in the Eagle Nebula (NASA, ESA and The Hubble Heritage Team, STScI/AURA).
