Below is a great article describing what I think are a lot of legitimate reasons why screenwriting and novel writing are different skill-sets.  

Novel writing is more detailed to give you visuals and screenwriting is  mostly dialogue and situations. It's up to the film director to create the visuals. 

Why Novelists Don’t Write Films

The book Gone Girl (left) and movie poster for Gone Girl (right)

by Lindsey Bahr | Entertainment Weekly
 October 23 2014 

Novelist Gillian Flynn has made Gone Girl a hit—not once, but twice. The film adaptation of her 2012 best-selling book has earned $77.9 million at the box office as of press time, taking the top spot both weekends since its release. The fact that Flynn wrote the film’s screenplay—which has garnered critical raves and could earn her an Oscar nod—lands her on a tiny list of authors who have successfully brought their books to life on the screen. (Only five have won Oscars for doing it.)

It’s a wonder more authors don’t make the jump to screenwriting, especially when studios are hungrier than ever for popular literary brands such as The Fault in Our Stars and The Silver Linings Playbook (not to mention, you know, The Hunger Games). So why don’t more novelists adapt their own work? Short answer: They’re usually bad at it.

”The two big differences between books and movies are pace and perspective,” says screenwriter and cohost of the popular Scriptnotes podcast John August (Charlie and the Chocolate Factory). ”Novels can luxuriate in internal moments of indecision and longing. Movies keep chugging along at 24 frames per second.” Flynn’s adaptation—which August says he ”desperately wanted to write”—soars because she was willing to trim the fat, cutting subplots and even characters.

”You start making the cuts that are really painful,” Flynn says of her kill-your-darlings approach. ”There are certain scenes that I would just hang on to. I knew, ultimately, they were going to go. I just couldn’t quite do it yet.” Through a series of drafts and five-hour phone calls with director David Fincher—”He very much likes to see the beginning, middle, and end of a scene”—the final product came together. ”There was something thrilling about taking this piece of work that I’d spent two years painstakingly putting together, taking a hammer to it, bashing it apart, and reassembling it into a movie,” she says.

Flynn’s agility, however, is uncommon. ”Usually I don’t want my authors to take a pass at writing screenplays because it’s a different skill set,” says literary agent Douglas Stewart, who represents Silver Linings author Matthew Quick. (Director David O. Russell earned an Oscar nomination for writing the adaptation.) ”A lot of authors want to do it, and I think it’s a rare case where an author should do it.”

Creative control aside, screenwriting holds financial appeal for authors. Rights to a novel are pretty cheap—optioning will often earn writers less than their initial book advance—but a screenwriting assignment can pay well into six figures. Yet an author pushing to adapt her or his own novel can backfire; insisting on right of first refusal before the development deal is finalized can put the project in permanent limbo. Studios, agents say, only want authors involved in the screenplay if they’re A-listers. (And even then it’s risky: Jonathan Tropper’s recent adaptation of his novel This Is Where I Leave You elicited lukewarm reviews and failed to impress at the box office. That may not be his fault, but it doesn’t exactly put studios at ease.)

All that said, Gone Girl could be a harbinger of a shift in industry thinking on the matter. Eleanor & Park‘s Rainbow Rowell is writing the film adaptation of her beloved coming-of-age tale for DreamWorks Studios, while Beautiful Ruins author Jess Walter is co-writing a script version of his book with director Todd Field (Little Children). Could one of them be the next Gillian Flynn? ”The smart novelist writes the best book she can and lets the movie be the best movie it can be,” says August. ”There’s no victory in a faithful adaptation if the result is mediocre.”