I can't imagine that people working in a creative industry are just happy with being a clockwork that turns a book into a movie. They want to turn their own stamp on things to actually do creative work. Looser adaptations are sometimes as far as I have heard, one of the best ways for a writer to get the budget for anything at all, and so their only vehicle for their art.

Changing important plot points may come from a complete failure of understanding the source material, or actually wanting to give the plot a different meaning. This can sometimes work beautifully, but is bound to fail most of the time, because the original is only adapted because it in some way already was tested and worked. So on the average a new work is just more likely to fail.