I started playing tennis again, building on some basics I learned as a kid. Joined a local club, and was dumbfounded why no one was playing on beautiful weekend summer days. The sport has declined a little but still, where is eveyone?

Turns out all the action is at the organized league play with teams of 6, playing at home or travelling to nearby clubs. Through age groups and play modus anyone who can nominally complete a match skill-and fitness wise can join and contribute both to team success and social life surrounding the play. Through the shared activity you can connect, and meet for a match before joining people for non-tennis activities, before "making a friend as an adult".

So yes, the social structures are eroding rapidly and this will have major consequences for society, but there are still pockets left that you'll have to go out and find.