I still can't stop but think how stretched out the Hobbit films were, for an otherwise short book. That's a sin that stains forever with me. The Lord of the Rings films, do have their own flaws, but they're fairly contained and can be debated as to whether they were good or not. I have so much trouble excusing anything with the Hobbit though.
And I hope we never, ever see anyone tackle The Silmarillion. Simply because, there will be so much improvised fluff in that would-be film, that it'd make the Hobbit films blush.
I don't remember the timeline, but perhaps they jumped on the bandwagon while LotR was still fresh in everyone's mind, thinking they'd get rights to touch other Tolkien sources, and then were told "hell no" so had to use a few scraps and make it up as they went.