Why do people prefer to be in a group than loneliness?

That's not true dear Vinay, it's not the case with all the people as you said. It would depend on how and what frame of mind someone is going through, it would also change with age and involvement in terms of your niche, loneliness becomes unbearable when your mind is wandering but if you dig deep you would discover that there are a lot of people who love being lonely and it's not out of depression for sure but it is a sense of being self-sufficient. Now coming to your question of why people prefer to be in a group than being alone, so as I said earlier it's a sense of need, this need is enormous and has a various aspect to it, recognition, love, friends, competition, desire, etc, etc. It is applicable to the one in the manner he or she is adapting their own surroundings and individual progress.