When I was growing up, a good deal of these sorts of parties involved backyards, barbecues, and swimming pools. Rarely did we spend time indoors.
I remember when i was young going out on vacations with school friends and their parents for several days. Also similar to weekends at country houses. The trend was that a couple of parents liked to host the get togethers and didn’t mind 10 adults and 20 children of different ages running around the place
None of my friends have a backyard big enough to host a BBQ
This might explain the decline then, as with the price of housing going up less people can afford a place that has a backyard (let alone a pool!)