I believe it's because individuals like to work alone. Perhaps they think they can do the task better than others and don't trust their colleagues with it. As a result, they often have more work and probably more stress than their teammates.

Some people think team work is not efficient. They believe that teams often need a lot of meetings to plan work and making decisions can take a long time. This can take up a lot of time and some feel it's better to use this time to actually do the work. That's why they prefer to work alone instead of in a team.