For most people it's because they game when they can, and that doesn't always allow room for organising a group. If they've spent all day on the phone, working out complex problems or having to adopt an 'image' for a customer facing job, the very last thing they want to do is spend their evening on chat, working out puzzles or fitting a 'role' like a lot of group activities want them to. For a lot, gaming is their 'me time' when they just want to immerse themselves in something atypical to the life they've just spent half their day living, and that often means retreating to the pleasure of their own company. They spend their working lives being told how to think, act and behave a certain way, doing things they don't want to be doing and having to deal with people they don't like. Their leisure time should be spent doing the exact opposite.