And what are your qualifications to judge decent writing? Some ****ing people...they go pick up a copy of Return of the King that's abridged to hell with Viggo Mortensen on the cover and all of a sudden they are literary geniuses.
Asimov isn't AWFUL, but he's certainly not the storyteller I'd like him to be, personally.
In any case I think my point here was a no brainer, but I'll be specific if it'll help you to understand: If the movie had followed the book, it would have been a bunch of flat boring characters sitting around going "Hmm...what if robots didn't listen to us?!" in monotone and nothing else. And that would have made an extraordinarily SUCK movie...don't you think?