'No aspect of writing makes you rich' – why do authors get a pittance for film rights?

Joanne Harris has revealed that she was paid only £5,000 for the film rights to Chocolat. So why do some authors get the big bucks and some have to settle for the crumbs?
Read more: https://www.theguardian.com/film/2020/aug/27/no-aspect-of-writing-makes-you-rich-why-do-authors-get-a-pittance-for-film-rights?source=Snapzu