Even potluck parties tend to be better on average when someone or a few people are "in charge". In my experience, even when people are just getting together for dinner out, there are people who step up more to organize.

Are you sure there aren't certain people driving these "informal" parties?

In Brazil, you are expected to bring food or drinks when you are invited as a guest to a party. If you stay until the end, you are also expected to help clean up the place. Guests will often take over certain parts of the party without even asking, such as preparing drinks, taking care of the barbecue, serving people, or going to the store to buy more drinks.