Why?

I've been pondering a minor mystery today.  Why do some parents have such a difficult time separating themselves from their spawn?  I love my kids, but, you know, when they hit adulthood and wanted to lead their own lives, that was fine with me and the S.O.  We like visiting them, we enjoy seeing the grandkids occasionally, but we don't have any particular desire to live right next door to either daughter.  I tend to take the fact that they're managing to survive just fine without living in the same state as us as proof we succeeded in raising offspring who can fend for themselves. 

What prompted me to think about this was an e-mail from a sort of relative, my cousin's wife, telling me that she and her spouse had just purchased a new house, the one they plan to live in permanently as retirees.  It's something like 30-seconds away from one kid and spitting distance from another.  Why?  These are people who are in their 60s now, which means their kids are in their 40s or really close to it -- why on earth is this woman still clinging to them?  I don't get it.  Granted, they are the ones that are eventually going to pick her nursing home, but even so. . .  I always thought empty nest syndrome was something that hit some people briefly and then passed.  Apparently not.