Why? Well, we're living farther from them than ever before, and we're living healthier so we don't just sit around and wait for a visit... if we can afford it, we may be off traveling.
Our two grandsons come to visit about once a month, and the oldest one (11yrs old) loves to sit and play video games, so we have to make sure to drag him outside to play, help with the garden, etc. That family lives about 65 miles away and headed by a single mom who is always busy.
The other child is married, lives 300 miles away, but no kids yet, and most likely never due to health issues. We see little of him since his job is uncertain and they don't have much $$ or time. we visit them 2-3 times a year.
Families don't live together like they used to. My best friend lives with her married daughter and their kids, and grandkids. Gets pretty hectic there, but she loves it as long as she gets a break once in a while.
It's an interesting study though.