This is because a lot of them have social lives that revolve around the workplace, usually because they're still at their first or second place of employment and they don't have kids. When work leads to evening dinners and bar runs, it's a vital part of the social experience. When you work with a bunch of seasoned developers in their 30s and 40s, you find that they have taken ownership of their social lives and their priorities after hours often do not include you.