Oh yeah, graphic novel adaptations are definitely good way to make some cash. That's probably why so many of them are less than stellar. They have several things going for them from the POV of publishers:
1 ready-made story -- no need to pay for a writer
2 royalty free -- the overwhelming majority of these books made into graphic novels are "classics" and in the public domain now.
3 already recognizable -- there's really no need, nor point in advertising Austen or Dickens now is there?
That being said, I am a bit of a fan of these kinds of things and when done right they can really sparkle. Peter Kuper's adaptation of Kafka's The Metamorphosis is a perfect example of a graphic novel that compliments and even adds something to the original.
On a more mundane level, I find GN adaptations to be a great way to experience certain books -- especially some of those old, terribly long winded ones -- that I would probably never read others. The Mysteries of Udolpho, for example.