Well, ours always let us know (in fact we usually arrange their visits a week or two before,) because they live 15-18 miles away. It would be daft for them to come without warning, (or us to do it to them,) because we may not be in.
Also, I work from home - 18 hours a week, and DH works 27 hours a week, (not WFH,) so I could be busy working, and DH might not be in. He works nights too, so may be in bed!
I have a busy life too, and I have several hobbies, and friends, and am likely to be busy when they turn up. And they would not appreciate me and DH just rocking up as they are busy career people, with lots of friends, and hobbies, and a busy social life.
It's a different world now for many (than what it was pre early 1980s,) as many people live some miles away from their parents, many more women work, and many people have more hobbies and interests these days than 40+ years ago that involve leaving the house. And some people work from home now. So it's not feasible for people to just turn up when they feel like it with no warning, and expect to be entertained, and expect people to just drop everything they're doing - or planning.
I mean if my adult DC did turn up and I was busy, I wouldn't be angry or pissed off that they'd come unexpected, more a little disappointed that they have come all this way and I can't give them the time I would like to, and spend time with them. If they give me a heads-up, I could possibly re-arrange stuff ...