Turns out people at meetups often have very full lives already, and many are there for the business networking opportunities, not to develop an outside-of-work social life.
So there were LinkedIn connections made, and some transactional relationships. I did occasionally meet some people later one-on-one. (I shouldn't have said they didn't lead to further interactions; I'd forgotten these few). Good and interesting people. But they're not what I'd call friendships. Perhaps the seeds of one, if they went on for years.
To be fair, I wasn't looking for more social life either. I merely observe that friendships did not flow naturally from going to meetups, the way you seem to be suggesting happens readily.
On the other hand, a chat with someone random at a library in the same time frame did lead to friendship. As have many other life events.
What I'm suggesting here is that meetups are a great way to find people interested in similar things, but at least in my experience, I don't recommend them as a way to find friends if that's what you're looking for.