I've had this experience too, but in my case I can tie it to Scottish clannishness! Scots really do seem to like to research their extended family, very thoroughly.
In my paternal grandfather's line (being a Scottish clan), almost everything has been researched by a range of people so there really is not a lot left to do, until I get right back to my gggrandmother - a fascinating lady of French, Dutch and Irish extraction. Some family legends there, but the fact that she came from the West Indies makes researching very difficult!
As for my maternal grandmother's line, a second cousin of Scottish descent on her father's side is a genealogy nut and has cornered the market there too!
Still, I have plenty left to look for on my other two grandparent lines, as well as the West Indian lady! There is a London family in my paternal grandmother's line who resolutely remain a mystery.

There's another disappearing ancestor as well. As for the Irish -

MarieC