There are way more than a few. Many people here slab books and submit books to CGC. I think some people here forget that the vast majority of second-owner collections are still raw. There are plenty of guys wo built up second-owner collections in the 60s, 70s, and 80s who basically stopped buying but didn't sell and may never sell. Some of those collections will just be handed down and kept for sentimental value.
Even tough it isn't very valuable, I still have the coin collection that my great-great-grandfather put together by saving old and foreign coins that came into his saloon, which he had to close when Prohibition came. With some of these comic collections, the value of them might tempt the heirs to sell whereas the $1,000 I might get for that coin collection wouldn't mean anything compared to the sentimental value. Still, some of those second-owner comic collections might not be sold for decades. For some people, the sentimental value means that they won't sell at any price, especially if they're well off themselves.
The one thing that bugs me is that people still refer to Gerber's scarcity index when selling books. It should be obvious at this point that he basically pulled his numbers out of his rear end. Aside from a few books that were legitimately rare and easily identifiable as 9s or 10s, those numbers are basically meaningless. Even some of the white-space books are fairly common. I don't think he looked too hard for them.