No quote has done more of a disservice than this Field of Dreams quote. There are more startups, meetups, and corn field baseball diamonds that lay unused and empty because their creators thought that if they just built something great, people would naturally find their way to it.
There’s a lot of noise out there, and even quality products need to find a way to cut through it. When the Battlestar Galactica miniseries and television reboot launched, there were commercials and teasers, but also volunteer street teams at malls and 500,000 sampler DVDs given out to fans for promotion. Uber and Lyft give out tens of thousands of dollars of free rides in every new city they launch in. Taco Bell sent a "Tacodactyl” to deliver tacos to the homes of Youtube vloggers (it’s debatable if Dorito Tacos is a “quality product,” but the PR stunt was fantastic).
Your meetup is no different. There are a lot of things competing for your potential attendee’s time. Family, work, sports, book club, repainting the shed... not to mention other meetups. Future attendees not only have to hear about you, but they have to decide that their time is better spent with you than with their family, or at a different event, or sleeping, on any of the other things they typically do.