This I'll keep short and simple...

As VCRs are becomming a rare sight at the stores; same for the cassette tapes, people want to update their video collection to DVD.

Thing is not all VHS films are available on DVD as authentic release. For one its easy to do the research. Check out www. IMDB .com and www. AMAZON .com Either of the sites (sure there are others) that give explaination on if there is a DVD release of  an out-of-production or pre-production film. 

It is illegal for sellers to offer these videos, even if they are professionally made on non-purple DVDs, VideoCDs, Or have expired their trademark.  Most of all the old 'online video bank' websites have been shut down because of this. And the #1 problem with those discs was that the menus were disabled making navigation impossible.