The sad fact is that there are hundreds, if not thousands, of titles that have not been transferred to DVD, including many small, independent films that are masterpieces, such as "My Dinner with Andre" and "Wrestling Ernest Hemingway."
Check Amazon, as I have found some excellent deals on movies I never thought I'd see again, although I have had to settle for VHS.
Disney is funny with DVDs; but, in a sense, it is marketing genius. They "retire" DVDs after a certain period of time, then place them in a vault for "x" amount of years. After mega-demand, new generations, etc., they "release" them to the public for a short time, and then demand and frenzy start over again.