Referrals happen over text. Trust gets built over a drink. These meetups are where that happens.
A referral is a five minute favour. The reason someone's willing to give it is everything that happened before that moment, usually over food, drinks, or standing around after both.
The network works because people actually know each other, not because they're in the same group chat. Meetups are how strangers on a spreadsheet turn into people who'd vouch for you.