I was wondering the same thing as I went for a walk.
As kids, you got many social experiences, to try and make friends.
As adults, there's work and hobbies... for some they have a family as well.
As someone who's single with no kids, I'd say other than sharing a hobby is the best way. Otherwise, most adults I find have their friends and are kind of closed to bringing other people in. I find that a lot of people I "knew" from school got indifferent and are with their closer friends, and a lot seem to do that. So, I guess most longtime friendships either solidifies or collapses. Meanwhile, meeting new people, in everyday life, has become very tough.
The thing about family and kids, is that it's possible for kids to bring friends over and for parents to have a connection with other parents.
Not saying it is always recommended. It's tough when most people are wary of strangers... but I also became wary of others.
my 2 cents.