Middle aged American here. I go to more social gatherings than I have in any time since college.
I would add a second dimension to your stratification: People who have kids and those who don’t. Once you have kids who enter school age, there are more social opportunity than you have time for. Even many of the people who never threw parties before now host a few cookouts or birthday parties per year.
The events are based on people who attend the same school or whose kids are friends with your kid, so you’re not always hitting it off with all of the other parents as future best friends. However you do get exposed to a lot of new people and each new introduction increases your odds of finding people you connect with.
One realization is that it didn’t actually require having kids to start doing all of this. Inviting people over for a backyard cookout or a simple dinner was always easy. Easier without kids, actually. Most simply don’t make the initiative.
Another problem is the increasing burden that people create for themselves or their friends with social events: Expectations of bringing gifts, showing up with a bottle of wine that isn’t too cheap, dressing nicely, and the biggest event killer of all: Trying to find a date that works for everyone of the guests. Parent get together dispense with all of that. Everyone’s busy with kids. If you show up for 30 minutes in shorts and a T-shirt bearing no gifts and you have to leave early, everyone is just happy you came. A date is sent out and whoever is available can come. If not, we’ll see you next time.