When I want to gather friends together and don't feel like hosting, it's usually going to be on an evening; that rules out a lot of options already. The only things that are ubiquitous and comparable to bars for those purposes are restaurants, but it's a hassle asking everyone to have dinner at the same time, waiting for everyone to show up before ordering, etc. (Plus they're more expensive, and I'm more likely to have a friend who can't find anything there they'd like to eat.) So I default to bars.