That's me!
Why?
So many more titles are on DVD than streaming. Many many more.
Try watching anything by Rainer Fassbinder on streaming: does not exist, but are on DVD. But that number is dropping due to licensing issues, just like streaming. My DVD queue has movies saved in "unknown" that I've actually SEEN from Netflix back in 2002 when I first subscribed.
Examples of DVDs from Netflix ca. 2002-2005 that I watched which no longer exist:
Lumiere and Company
The Third Man
The House that Dripped Blood
Lies
Crimes of Passion
Sadly my "unknown availability" list is growing.