Perhaps it's a trend, since Alexander Payne wrote JP3. Yes, as someone above said, they do it for the cash to fund their own films.
Probably a take-the-money-and-run gig. Doubt the script is going to get nominated for an Oscar.

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"Perhaps Jim Jarmusch can contribute to the next Bond film - With Tom Waits doing the song."
LOL!

Seeing Sayles' name on Jurassic IV gives me a good feeling about the movie. It should have that particular style of dry wit that one sees in his scripts - And I believe it would mix well with a big budget action movie. Let's just hope the director and cast understands the new style.
Perhaps Jim Jarmusch can contribute to the next Bond film - With Tom Waits doing the song.
"They're making a new film of Moses.It's not finished,but the baby looks great in the rushes"

John Sayles often works as a "script doctor" for big Hollywood films (sometimes uncredited, sometimes not). This is where he makes a good portion of the money he then uses to make his own films without any big studio interference.

I don't know. He wrote those monster movies for Corman, so it would be interesting to see what he would do with a much bigger budget.