In D/P, the charactors have no manors and are snotty brats, busting into random peoples homes and running around. But back in the olden days, people had manors, and knew not to trash peoples homes... See? Realistic. Kids 10 years ago had more manors than kids now-a-days. Pokemon it just trying to teach us all a leason. Although I've lost myself in all this and I forgot what it was.
Edit: Oh yeah, the topic... It's cuz the ships crashed and no one lives there. And there's less reason to run in someone's house than when you run to explore something.