Screenwriters are, with few exceptions, freelance workers.
They work contract to contract, with no guarantee that the contract they are currently writing will not be their last. Relatively few writers work consistently contract to contract – a median screenwriter has one year in five in which s/he earns no income at all from Screenwriting.
In addition to this profound instability, screenwriters are often poorly paid and earn relatively little from secondary payments (about 10% of their audiovisual income). Very few have earnings from the use of their work online.