“Secrets of the Script: What Hollywood Won’t Tell You About Unsolicited Screenplay Submissions”
Today, this is predominantly how movies are made. Studios work more as distributors, beyond some of their tentpole franchises. This new Hollywood system allows screenwriters to work on a freelance basis rather than under long-term studio contracts.

The Screenwriting Boom of the 1980s and 1990s
The 1980s and 1990s saw the rise of the spec script market. It started with the impending 1988 Writers Guild strike, which had studios clamoring for screenplays. After the strike, studios needed scripts to ramp up production once again. Since screenwriters had been on strike, they had a plethora of screenplays written on spec (under speculation that they would be sold) during the work stoppage.