Just showing up is something that has definitely gone out of fashion. I have family I barely see because meeting up requires advanced notice. Even then it feels like an inconvenience, hence, other than weddings and funerals, I don't bother.
I personally have an arrangement with some friends: they can call me or pop over any time… but they have to accept I might be busy, and vice versa. So far it has worked because we’re all too lazy to be doing anything other than hanging out.
A few theories:
We prioritize unimportant stuff over people. Examples: going to the gym, grocery shopping
Too much ceremony around the actual meeting. Your place gotta be tidy, you gotta be a good host etc. I have neighbors I really like who moved in ~6-8 months ago. We had them over twice and visited their place once. Each time each party made a huge effort. My wife made us clean our place for the better part of the day and every time we visited someone clearly had spent hours cooking and prepping stuff. Nobody wants to do this all the time. I even brought this up explicitly that we all gotta chill the f out about being super hosts. Everyone nodded and I am sure whoever hosts next time won't put much less work in.
Cultural change. I don't even know how to get to the place where I would just walk to my neighbor to "hang out". It feels like everything these days needs to have a reason and a plan. And this is in the rare case where you actually could walk over and don't have to drive 20 minutes to find out your friends took their kid to swim class.
