That said, there are many types of training you can't do without the right equipment, so it's at least a bit more than just wanting to be with others. Strength training/powerlifting/olympic lifting all come to mind. It's often really difficult to keep a rack, barbell and hundreds of kilos of weight at home, and obviously not something people had just lying around in case of an emergency.
There is certainly a social aspect to it as well, I love training with a couple of friends, the atmosphere can't be replicated at home. But like I said, I've abstained, and I would have been hopeful others would have to.