The LW asks: "Is this a sign of the rudeness that is so pervasive in society?" meaning not being invited in if you just drop by someone's house.
I'd say it is more a change in the etiquette of dropping in--you don't drop by someone's house unless you are prepared for either the entire family being out or for not being invited in, because the family simply doesn't have time, right then and there, to visit.
Growing up in the 1960s, except for work and school, there was usually someone home. Mom did most of the errands during the day on weekdays, and afternoons after school and on weekends, we were home a lot. There'd be Little League games and once or twice a month we'd have an outing to a movie or museum or historical site, but that was about it.
If someone dropped by unexpectedly, one of my parents would have the time to stop what they were doing and sit down and have a cup of coffee and a chat. I don't remember any drop-ins staying for a meal, though.
Things have changed. Children are more scheduled than my mom used to be. More women are working outside the home, which means that the meetings my mom used to go to during the day are now held evenings or weekends. There's simply more things to do with your free time, as well.
Sure, you can drop by unannounced. But to expect families to keep their kids home from various activities because you've dropped by, or to cancel a family outing because you've dropped by, or to cancel a rare family meal all together because you've dropped by, is expecting way to much.
It's not rude to tell an unexpected visitor that you can't visit right now. It is rude to drop by unannounced and expect that the family should stop everything to host you.
In a slightly related story, my cousin got married in 1974. The newlyweds moved into an apartment a few miles from her parents' home. At some point, either she or her new husband commented to her parents that they never dropped by, and indicated this would be a nice thing to do.
So my aunt and uncle dropped by one evening a few weeks later. My cousin had just gotten out of the shower and her hair was dripping wet. Her husband was in his boxer shorts, drinking beer out of the bottle (something that my aunt did not approve of) and cheering on the baseball game on TV.
Everyone was quite embarrassed and no mention was made of drop by visits ever again.